Against manifestationalism

I recently read Asay and Bordner's (A&B) interesting paper "A modest defense of manifestationalism". Last week I discussed a stricter form of constructive empiricism which holds that we should believe that our best theories correctly describe not observable phenomena but only observed phenomena. A&B explore the same view under the name "manifestationalism". More precisely, they given the following taxonomy:

Scientific realism (SR): Science aims to give us theories that are literally true; acceptance of a theory involves the belief that it is literally true.

Constructive empiricism (CE): Science aims to give us theories that are empirically adequate; acceptance of a theory involves only the belief that it is empirically adequate.

Manifestationalism (M): Science aims to give us theories that are comprehensively manifestly adequate; acceptance of a theory involves only the belief that it is comprehensively manifestly adequate. (A theory is manifestly adequate just in case it correctly describes all observed phenomena.)

I have already mentioned that I don't think it's particularly helpful to frame the realism debate in terms of the aim of science. The interesting position is that of the manifestationalist who believes that scientific theories correctly describe only observed phenomena, and who is agnostic about the unobservable and the observabled-but-unobserved.

A&B clarify that manifest adequacy is not simply truth to the phenomena observed so far, but truth to all phenomena that have ever been or will ever be observed, including those observations that take place outside a scientific context (this is what they mean by comprehensive manifest adequacy). This clarification is important. Belief in nothing more than what has been observed so far entails radical scepticism about the future, a position which is not just implausible but probably psychologically impossible to adopt. M avoids this radical scepticism. M endorses the belief, for example, that I will continue observing my computer screen for as long as I sit here typing; that when I go downstairs to get some food I will observe some bananas in the fruit bowl; etc.

The case for manifestationalism

A&B do not attempt to present a conclusive case for M. They merely try to show that it is not absurd or irrational, but a serious contender in the philosophy of science. I will argue that they fail. Much of A&B's paper is devoted to answering a criticism of M based on the aim of science, which I discussed briefly in my previous post; I will focus on their arguments for M, of which there are three:

1. One possible argument for M appeals to van Fraassen's "vulnerability criterion" of belief, expressed in this passage:

If I believe the theory to be true and not just empirically adequate, my risk of being shown wrong is exactly the risk that the weaker, entailed belief will conflict with actual experience. Meanwhile, by avowing the stronger belief, I place myself in the position of being able to answer more questions, of having a richer, fuller picture of the world, a wealth of opinion so to say, that I can dole out to those who wonder. But, since the extra opinion is not additionally vulnerable, the risk is — in human terms — illusory, and therefore so is the wealth. ... What can I do except express disdain for this appearance of greater courage in embracing additional beliefs which will ex hypothesi never brave a more serious test?

Given two theories that are equally vulnerable, we should believe the logically weaker theory. Van Fraassen uses this to argue for CE, but A&B claim that this argument is really an argument for M, not for CE. We will only ever be able to test our theories against those phenomena that are actually observed; we will never obtain any evidence that could decide between two rival manifestly adequate theories. If I believe a theory to be empirically adequate and not just manifestly adequate, my risk of being shown wrong is exactly the risk that the weaker, entailed belief will conflict with actual experience... M and CE are equally vulnerable and M is logically weaker than CE. So we should accept M.

Of course, the force of this argument for M depends on whether we accept van Fraassen's vulnerability criterion of belief. I won't discuss the case for or against this here. Another question is, are CE and M equally vulnerable? Once the end of science occurs, and all the observations we will ever make have been made, the answer is clearly "yes". However the situation is somewhat trickier from our current position. We can see the problem clearly by asking, exactly which aspects of our best theories should we believe? It's easy for the constructive empiricist to answer this. CE tells us to believe what our theories tell us about the observable world. But M tells us to believe what our theories tell us about the observed world - observed in the past, the present, and the future.

The obvious difficulty here is that we don't know what observations will be made in the future. Consider the following case. My friend Frank has an apparently normal chicken egg. Nobody has ever observed the interior of this egg. Frank decides to put the egg in a box and throw it into a volcano, so that nobody will ever observe its interior (suppose this is part of some bizarre religious ritual). Well, if I accept M, then I must be agnostic about what it in the interior of the egg. Its interior will, in all likelihood, never be observed. But then suppose that while carrying the egg to the volcano, Frank drops it and it breaks, revealing the yolk and egg white.

The point, in the context of the "vulnerability criterion" argument for M, is that since we don't know what will be observed in the future, it's not at all clear that, at the present time, CE and M are equally vulnerable. The manifestationalist believes only that theories are true for what will be observed - if we don't know whether X will be observed, or we have good reason to believe that X won't be observed, then we must be agnostic about X. For this reason the manifestionalist, but not the constructive empiricist, must be agnostic about many things that will ultimately end up being observed. CE is therefore riskier than M, and this pushes us towards CE.

2. A&B suggest that M better captures the spirit of empiricism, which counsels us to limit belief to what in principle to tested against experience. The observations that we can make are constrained by our spatiotemporal location. CE involves belief in entities and processes billions of light-years distant or billions of years in the past, that we could not possibly observe. M offers a more modest position based on the specific position of humans in the universe and the epistemic limits entailed by this.

With this argument, I think that A&B have identified a genuine problem with van Fraassen's formulation of CE, but other versions of CE are less vulnerable to their criticism. We simply need to reconceive what is meant by "observable". I offer one suggestion in this post. The constructive empiricist hold the observable is what could in practice be detected by the unaided senses, given the technology of the time; and no feasible technology currently available would allow us to travel to distant quasars. This conception of observable is of course not precise - there is no absolutely clear line between the observable and unobservable - but this is equally true of van Fraassen's way of drawing the distinction.

3. Finally, A&B argue that the aim of an enterprise should be related to "what an ideal community of practitioners of that enterprise would accomplish under ideal circumstances." The aim of an enterprise should be attainable at least in ideal circumstances. A&B then suggest that the ideal scientific community is a community of people just like us, with the same sensory capacities and occupying the same spatiotemporal location in the universe, but who are devoted exclusively to the development of science. They meticulously and flawless record every observation. They generate theories that perfectly fit the observations. What will the final product of this community be? They will have a theories that are manifestly adequate - but not necessarily true or even empirically adequate. Rival empirically adequate theories will fit the data equally well. Even in ideal circumstances, there is no guarantee that we would produce empirically adequate theories, but we would produce manifestly adequate theories. This favours M.

I have two objections to this argument. First, why should the aim of an enterprise be decided by what an ideal community would achieve? Consider games with impossible aims. The aim of Klondike solitaire is to move all cards to their foundation piles; however, this is not possible even in principle for about 20% of deals. An ideal community of Klondike solitaire players could not move all the cards to their foundation piles in those deals - nevertheless, the aim remains the same.

Second, why suppose that there is a definable "ideal community" of scientists? A&B suppose that the ideal community is limited to our place in space and time; but why? Surely an even better scientific community would be one that existed as soon as complex life was possible, and had been meticulously collecting observations ever since. Would all members of the ideal community have perfectly functioning sensory capacities, e.g. 20/20 vision? Perhaps they would have superhuman capacities, 20/10 or even 20/1 vision perhaps. It isn't at all clear what counts as ideal in this respect.

It's clear that the ideal community will need some superhuman capacities. A&B suggest that ideal community would devote all their time to science, and they record every observation they make. Neither of these things is possible for humans. We need to devote time to other enterprises such as eating food, and we wouldn't be able to record every observation even if we tried. There are simply too many observations, and in any case, a person could not record observations while at the same time generating theories to account for those observations.

The problem for A&B is that in order to ensure that the ideal community generates manifestly adequate theories, they must suppose that this ideal community is capable of feats that go far beyond human capacities - indeed, feats that go far beyond the capacities of any conceivable organism. But then why not say that the ideal community is one that observes every observable? This community would produce empirically adequate theories - and this would give us an argument for CE.

What's wrong with manifestationalism?

There are many objections we might raise against M. I want to outline what I regard as a knock-down argument against it. Consider again Frank and his egg. We saw that Frank broke the first egg. So now he goes and out and purchases a second one, which again he intends to put in a box and throw into a volcane. This time, he takes many precautions to avoid breaking the egg, such as wrapping it thickly with bubble wrap. If I accept M, I must be agnostic about the interior of the egg, since I have good reason to believe that this will never be observed.

But now suppose that on the trip up the volcano, Frank gets hungry, and decides he wants to eat the egg. Well, now I have good reason to believe that Frank will observe the interior of the egg, and since our best theories tell us that he will observe a yolk and egg white, I should believe that he will observe a yolk and egg white. Of course, this example can be run in reverse as well, where Frank initially intends to eat the egg, and then intends to throw it into the volcano: in this case, I first believe that he will observe a yolk and egg white, and then I must give up that belief and become agnostic.

This is of course an absurd conclusion, absurd enough in my view that it's enough in itself to remove M from serious consideration. One response to this is to hold that we should remain agnostic about anything that has not yet been observed - even if we have good reason to think that Frank will open the egg, we should remain agnostic about the interior of the egg until he actually does it. But this entails radical scepticism about the future, which is no less absurd; and as I noted earlier, M was supposed to avoid radical scepticism.

Ultimately, it seems to me that A&B fail to show that M is a serious contender. There are very serious, very obvious problems with M, and no plausible arguments in its favour.

Constructive empiricism and the past

Derek Turner argues, in chapter 7 of Making Prehistory, that van Fraassen's constructive empiricism entails radical scepticism of the past. Here is Turner's argument:

(P1) All of our knowledge is limited to that which we can observe.
(P2) We cannot observe things which no longer exist or events which occurred in the past.
(C) Therefore, we cannot know anything about past things and events.

The constructive empiricist must suspend belief in dinosaurs, the Black Death, WWII, even what she had for breakfast this morning. Turner takes this conclusion to be far too radical, hence a good reason to reject constructive empiricism. I agree with him that such radical scepticism should be avoided, but I'm not convinced that the constructive empiricist is committed to it.

It seems clear that the constructive empiricist is committed to (P1), since this merely expresses the constructive empiricist's scepticism about unobservables. Van Fraassen would presumably want to reject (P2), but as Turner argues, it's not clear how he could do so. The natural response for van Fraassen is to say that some past entities such as dinosaurs were such that, if they were around today, we would see them. So dinosaurs are observable. The problem with this move, as I discuss in this earlier post, is that we may as well also say that if we stepped into a miniaturization machine, we would see bacteria.

In any case, the interesting question is not whether past things and events are observable per some definition, but whether there is good justification for believing in past things and events. My point is, we can surely draw the observable/unobservable distinction so that some past things and events count as observable - but similarly, we can draw that distinction so that electrons and neutrinos count as observable (we might say that to observe something is simply to detect it either with the senses or with instruments). In practice, dinosaurs are just as inaccessible to us as electrons and neutrinos - indeed, as Turner argues, dinosaurs are if anything even less accessible, since if electrons and neutrinos do exist we might interact with them and manipulate them instrumentally.

The constructive empiricist must accept (P1) and (P2), and the conclusion straightforwardly follows. However, the significance of the conclusion depends on how we interpret the word "we". Turner seems to assume that "we" refers to presently existing humans. I think we should resist this interpretation. To explain why, consider this very similar argument, which suggests that constructive empiricist to an even more extreme form of scepticism:

(P3) All of my knowledge is limited to that which I can observe.
(P4) I cannot observe anything other than my immediate surroundings.
(C) Therefore, I cannot know anything abuot that which is not in my immediate surroundings.

The problem with this argument is that science is fundamentally a human and communal activity. Perhaps there could in principle be one lone scientist, who correctly applies the scientific method (whatever that is) but who never communicates with any other people - clearly, however, her investigations wouldn't get very far. Every scientist relies on the testimony of others, and although testimony is often unreliable, there are circumstances where we can trust it. Science depends on this because a single scientist can't test everything. The vast majority of beliefs that a scientist holds even about the observable world will never have been tested by her. Speaking as a layperson, I've never observed the planet Neptune. I rely on the testimony of other people, who claim to have seen it through their telescopes. We work as a community to generate scientific knowledge; if you reject that community aspect, you undermine the very conditions of modern science.

Observability is relativized to the epistemic community. Who counts as part of that community? Not just contemporary people, but people in general - past, present, and future - and all their observations count. The Black Death was not simply observable, it was actually observed by hundreds of thousands of people, and we have numerous reports of it, many of which are reliable. So we should believe that the Black Death occurred. Of course, the people who observed the Black Death no longer exist. But why would that matter? Science is something that develops across many times and places. Indeed, building and testing theories can only be done over time, sometimes only over literally decades.

My point is that the existence of human communities over time, and certain conditions under which we can reasonably rely on the testimony of others, is simply taken for granted in any philosophy of science, because if this assumption rejected there is no reason to think that anything resembling science actually takes place. Having made this assumption, there is then an additional question about what else we should believe: for example, does science require us to extend belief beyond human observation?

Of course, a question remains about what justifies this assumption. Why should I believe that other people exist, that I can sometimes rely on what they say, etc? This questions arises when we "step back" from philosophy of science and it is, of course, a familiar question of philosophical scepticism. Perhaps it will turn out that whatever justifies belief in the existence of other people will also justify belief in some unobservable entities such as electrons. This would refute constructive empiricism, or at least it would make the constructive empiricist's selective scepticism of unobservables arbitrary. But in principle, the constructive empiricist need not be committed to scepticism of human history.

What about prehistory? Can the constructive empiricist believe in dinosaurs? On this point, I'm inclined to agree with Turner: constructive empiricism implies total scepticism about the distant past. I don't, however, share Turner's feeling that this is an especially extreme position. Indeed, it's curious that Turner regards such scepticism as so repugnant, because he devotes a great deal of his book to defending an epistemic asymmetry thesis that we have better access to the microscopic and microphysical than to the distant past; as he puts it, "there is a rough sense in which we can know more about the tiny than about the past" (pg 25). However, Turner is rather sanguine about the constructive empiricist's scepticism of microphysical unobservables; he sees this as a fairly modest feature of the theory. If we have better access to the tiny than to the past, and if scepticism of the tiny is at least reasonable (though perhaps not rationally compelled), why would scepticism of the past be repugnant?

Observable or observed?

In my previous post I discussed some difficulties with the observable/unobservable distinction in constructive empiricism (CE). Here I'll focus on a related problem, the distinction between the observable and the observed.

Extreme constructive empiricism

In accepting that our best scientific theories correctly describe all observable phenomena, CE goes far beyond the empirical evidence. Most of what is observable will never be observed. Consider this position, which I will call "extreme constructive empiricism" or "ECE" for short: our best theories are true for the observed phenomena, but we should be agnostic about everything else. Why not adopt this more sceptical position? There are some obvious benefits to ECE. In holding that science delivers truth about the observable, two common arguments for antirealism are unavailable to CE:

(1) The pessimistic induction. Put briefly, this argument goes as follows: (P1) Most scientific theories accepted in the past have been rejected; (C) so, most currently accepted theories will be rejected in the future.

However, every case in which claims about unobservables have been rejected has also involved rejection of claims about observables. Those theories that we rejected in the past were rejected because they conflicted with observations. If our current theories are going to be rejected in the future, this will presumably be because they confront new observations that contradict them. So no antirealist who takes current accepted theories to be true for observable phenomena could endorse the pessimistic induction. Obviously this difficulty doesn't arise for the ECEist who only takes those theories to be true of the observed phenomena.

(2) The underdetermination argument. This argument rests on the claim that for any theory that correctly describes observable phenomena, there are numerous other theories that also correctly describe observable phenomena but that make different claims about unobservables. Again, it is easy to see that this can be run against CE, because for any theory that correctly describes what has been observed, there are numerous other theories that also correctly describe what has been observed but that make different claims about observable-but-as-yet-unobserved phenomena.

So what is the justification for belief in the observable over the merely observed? I will discuss two arguments.

The aim of science

First, Van Fraassen and Monton argue that CE is an attempt to describe the aim of science, with an assumption that the behaviour of scientists is rational. If the aim of science were truth about the merely observed, rather than the observable, then:

there would be no scientific reason for someone to do an experiment which would generate a phenomenon that had never been observed before. But one of the hallmarks of good scientists is that they perform experiments pushing beyond the limits of what has been observed so far.

Suppose I develop a theory of galaxy evolution that accommodates all that has been observed so far. I could derive further predictions from this theory, and then probe the sky to check if those predictions are accurate. But if the aim of science is simply truth about what has been observed, why bother? My theory already achieves this aim; it is therefore as good as any theory could possibly be. It would be unreasonable to risk turning an perfect theory into an imperfect one by making further observations that could contradict it. By contrast, in holding that the aim of science is truth about the observable, CE accommodates the fact that scientists spend so much time generating new observations.

There are a couple of things to say about this. First, assuming that it even makes sense to talk of the aim of science as a whole (as opposed to the aims of particular scientists or particular research groups), why would we suppose that science has only one aim? Two others goals that science plausibly aims at are the discovery of new phenomena and the manipulation and control of natural phenomena. If science aims at discovery, that immediately makes it rational for scientists to make new observations. If science aims at control and manipulation, new observations are always worthwhile because the more we know about the world, the better we are at controlling and manipulating it.

Second, we must distinguish the aim of science from the epistemology of science. Just because science aims at empirical adequacy, it doesn't follow that we should believe that our best theories are empirically adequate. Perhaps science sometimes falls short of its aims. After all, we all know that science has developed over time, and we hope that it will continue to develop in the future, that in the future it will be closer to achieving its aims than it is today. So the question remains, why suppose that our scientific theories save the observable phenomena, rather than merely save the observed phenomena?

Epistemic risk

A second argument for belief in the observable is hinted at by van Fraassen in his contribution to Churchland & Hooker's Images of Science. Although this is stated as an argument against realism, we can use the ideas here to support CE over ECE:

If I believe the theory to be true and not just empirically adequate, my risk of being shown wrong is exactly the risk that the weaker, entailed belief will conflict with actual experience. Meanwhile, by avowing the stronger belief, I place myself in the position of being able to answer more questions, of having a richer, fuller picture of the world, a wealth of opinion so to say, that I can dole out to those who wonder. But, since the extra opinion is not additionally vulnerable, the risk is — in human terms — illusory, and therefore so is the wealth. ... What can I do except express disdain for this appearance of greater courage in embracing additional beliefs which will ex hypothesi never brave a more serious test?

The realist believes everything the CEist believes, but the realist also holds additional beliefs that cannot face any additional tests. As Kukla (pg 98) puts it, van Fraassen seems to adopt a "vulnerability criterion of belief". Two theories are equivalently vulnerable when they are disconfirmed by precisely the same observations. Van Fraassen holds that if two theories are equivalently vulnerable, we should believe the theory that is logically weaker.

CE is obviously riskier than ECE, since the CEist is committed to beliefs about what has not yet been observed whereas the ECEist is agnostic about this. The two positions are not equally vulnerable. Of course, this is not yet an argument for CE. Indeed, if anything this looks like an argument against CE: intuitively, if a belief is riskier, if it is more likely to be wrong, then that is a reason not to hold that belief.

But notice that if our goal is simply to minimize the risk of holding false beliefs, then we should become radical sceptics and believe nothing at all (or perhaps we should believe a few propositions that we think are beyond doubt, but certainly we should believe nothing substantial about the world). Clearly we have other goals. There are, broadly speaking, two types of epistemic error: type I errors consist in believing what is false; type II errors consist in failing to believe what is true. Attempting to avoid one type of error pulls us in a different direction than attempting to avoid the other type, and an important question in epistemology and philosophy of science is how to balance these two goals.

Here is a simple suggestion: We avoid type I errors through the vulnerability criterion of belief; don't hold beliefs that go beyond what can be tested empirically. We avoid type II errors by holding the strongest possible beliefs given the vulnerability constraint. This is exactly what CE does. ECE makes very severe type II errors: to be agnostic about everything beyond what has been observed entails total agnosticism about the future, and this would be disastrous even assuming that it's psychologically possible. ECE undoubtedly commits fewer type I errors than CE, but the price in type II errors is simply too high.

Observables in constructive empiricism

Constructive empiricists such as Bas van Fraassen hold that we should take our best scientific theories not to be true but merely empirically adequate, where a theory is empirically adequate when what it claims about observable phenomena is true. We should believe what our best theories tell us about the observable world, but remain agnostic about any unobservable phenomena they postulate. For the constructive empiricist, something is observable just in case it can in principle be detected by the unaided senses. So we should be agnostic about electrons, mitochondria, the strong nuclear force, the tricarboxylic acid cycle, etc.

Obviously, the cogency of constructive empiricism depends on whether the observable/unobservable distinction can be made to work. Van Fraassen treats observability as a property of entities. Some types of entities are such that they produce certain changes in our perceptual systems, provided we have the right kind of relation to them; these entities are observable. For example, Jupiter's moons are observable. From Earth, of course, we require telescopes to detect them, but they are in principle detectable with the unaided senses because we could travel to them, and then they would impinge on our senses in certain ways. So van Fraassen isn't agnostic about Jupiter's moons; he believes they exist.

The question is, how should the "in principle" be interpreted? What kind of relation to an entity is the "right" kind of relation? Van Fraassen, as far as I know, is fairly dismissive of this kind of problem. He thinks that "observable" is a vague predicate so there is no precise distinction between the observable and the unobservable, but this is no threat to constructive empiricism because even with a vague boundary we can still specify entities that are unambiguously unobservable. It seems to me, however, that without an answer to those questions, it isn't at all clear that there are any unambiguously unobservable entities - or at least, that the class of them becomes so small that constructive empiricism fades into realism.

Van Fraassen is happy to count distant astronomical objects such as quasars as observable, because we would detect them unaided if we travelled billions of light-years in their direction. Similarly, past entities such as dinosaurs are observable, because we would detect them unaided them if we went back into the past. The observable is not just what is detectable unaided given current technology, as Jupiter's moons are. We can also imagine technology that violates the laws of physics, that can take us millions of years into the past or billions of light-years distant.

There are various problems with this treatment of observability, but the constructive empiricist in particular surely can't accept it. Once we're allowed to imagine totally speculative technology, we might say that in principle I could see a bacterium with my unaided senses, because I could enter a miniaturization machine and be shrunk down to the bacterium's size. Indeed, it's worth noting that miniaturization appears to be more plausible than travelling to the most distant quasars, since miniaturization doesn't violate any laws of physics as far as I'm aware. So on this view, a whole host of phenomena become observable and the distinction between constructive empiricism and realism fades. One fix is to say:

(A) The observable is what is detectable with the unaided senses given current technology, here in 2017.

But now consider the situation for astronomers working, say, 300 years ago. Given the technology at the time, both Jupiter's moons and bacteria were equally inaccessible. It later turned out that space travel was feasible whereas miniaturization was not, but surely there was no justification available in the 1700s for expecting technology to develop that way. If we accept (A), then the constructive empiricist has the curious conclusion that, in the 1700s, it was reasonable to believe in Jupiter's moons even though there was good reason to think that Jupiter's moons were unobservable in the same way that microbes were unobservable.

A similar problem arises when we consider future technological developments. Maybe we will in fact develop miniaturization machines and people will directly interact with microbes. If observability is determined by current technology, then microbes will still count as unobservable and therefore our miniature descendents should remain agnostic about microbes. With these points in mind, it seems we have to drop (A) in favour of:

(B) The observable at time t is what is detectable with the unaided senses given the technology at t.

However, technology can regress. If a nuclear holocaust occurs and we lose our ability to construct telescopes, then Jupiter's moons will become unobservable again. The constructive empiricist would have to conclude that we must once again be agnostic about the existence of Jupiter's moon. To avoid this conclusion we can modify (B):

(B*) The observable at time t is what is detectable with the unaided senses given the technology at t and any time before t.

This doesn't seem to face any obvious problems. However, an interesting consequence of (B*) is that it can be used to strengthen the case against the constructive empiricist by supporting an "optimistic induction" argument for realism about unobservables. This is because (B*) allows that unobservable entities can become observable as technology develops. Briefly, the argument is that numerous entities postulated by our best theories were initially unobservable but later became observable, thus vindicating those believed in such entities; so we can similarly expect that those who believe in currently unobservable entities will be vindicated in the future if the technology develops to make those entities potentially observable.

Jupiter's moons were once unobservable. Over the past few decades we have sent machines to Jupiter, and we could probably even send a person there, though that might be a waste of time and money. In any case, Jupiter's moons clearly do exist. The astronomers of the past who believed in these objects have been vindicated. Numerous other examples can be found from astronomy: (a) Other planets and moons and their properties. We can now see directly that the Moon's surface is disfigured by craters, that Venus undergoes phases similar to the Moon, that Mars has a tenuous carbon dioxide atmosphere, etc. (b) The true properties of the Sun, such as its size and sunspots. (c) The asteroid belt. (d) Comets. (e) Certain properties of the gravitational force. We feel a particular gravitational force on the surface of the Earth; now we can ascend to space in rockets and directly feel that the force is lesser there. (f) The shape of the Earth: predicted to be an oblate spheroid on theoretical grounds, its shape can now be directly seen.

Of course, this is an inductive argument, and van Fraassen rejects induction. However, as far as I can tell, this is not a necessary feature of constructive empiricism. In any case, anybody who accepts induction has an additional reason to be wary of constructive empiricism.

Philosophy of extraterrestrials: an unfairly neglected subject

Philosophers have devoted very little attention to the subject of extraterrestrial life. Cirkovic, Kukla, and Lamb are the only recent philosophical monographs on the subject that I'm aware of. The lack of philosophical engagement with the extraterrestrial question is puzzling for several reasons:

(1) As is demonstrated by Dick and Crowe, there is a long history of philosophical discussion of extraterrestrials. The early atomists were optimistic about extraterrestrials. They proposed an infinitely large universe, consisting solely of "atoms and the void", with different arrangements of atoms producing the diversity of objects we see around us. On this view, our planet and the organisms inhabiting it were created by chance collisions of atoms. Given this metaphysics, we can expect that other worlds with other living creatures will have arisen elsewhere. Lucretius, for example, says (quote taken from Kukla):

it is in the highest degree unlikely that this earth and sky is the only one to have been created … This follows from the fact that our world has been made by nature through the spontaneous and causal collision and the multifarious, accidental, random and purposeless congregation and coalescence of atoms whose suddenly formed combinations could serve on each occasion as the starting-point of substantial fabrics – earth and sea and sky and the races of living creatures.

Following the Copernican revolution, the scholarly discussion of extraterrestrial life snowballed, and optimism was the dominant position. There were two primary justifications for belief in extraterrestrials. First, most scientists accepted the Copernican Principle, which claimed, to put it simply, that there is nothing special about the Earth (versions of this principle are still accepted today). The stars are suns just like our own; and all areas of the universe are subject to the same laws of nature. Thus it was expected that planets similar to the Earth would be prevalent throughout the universe. Studies of other solar system bodies seemed to confirm the similarity to the Earth: astronomers observed clouds on Jupiter and mountains on the Moon. Second, abundance of extraterrestrial life was held to follow from religious commitments. God would not be wasteful; he would not have created a vast universe filled with stars and planets for no reason. Obviously such other worlds were not created for the benefit of humankind, so their purpose must be to house other intelligent species. Many scholars, such as William Herschel, went as far as to suggest that the Moon, the comets, Saturn's rings, and even the interior of the Sun, were inhabited.

Against this optimism were those who held that extraterrestrial life, even if it does not explicitly contradict anything in the Bible, is at least in some tension with a religion that is structured around humanity's unique experience and history. Should we suppose that Jesus died and was resurrected on an infinite number of planets? This seems absurd. Or perhaps extraterrestrials have not fallen and so are in no need of redemption? But now we are supposing that the Earth is uniquely evil. Along with these religious concerns, by the mid-1800s scholars such as William Whewell were emphasizing scientific studies showing significant differences between the planets, such as spectroscopic studies of the Moon that revealed its lack of appreciable atmosphere. Philosophers engaged in a great debate about extraterrestrials throughout the 19th century.

(2) The scientific study of extraterrestrial life, which is part of the field of astrobiology, is a highly interdisciplinary field currently undergoing rapid progress, however its conceptual foundations have yet to be fully developed. For exmaple, what is the status of principles such as the anthropic principle and the aforementioned Copernican principle, and how should they be applied? How much can we learn from the Drake equation? So far, much of this work has been left to the scientists themselves. Philosophical study of extraterrestrials will allow philosophers to work closely with an increasingly powerful science, and on a topic that is of great interest to many people even outside of academia.

(3) The problem of extraterrestrials is connected with a variety of topics in philosophy of biology, philosophy of language, philosophy of mind, ethics, and so on. For example:

-- One of the first questions that arises in the search for extraterrestrial life is, what exactly are we looking for? What is life? This is, of course, a central problem in philosophy of biology.

-- Assuming that life does exist on other planets, how similar should we expect it to be to life on this planet? There has been a great deal of debate in philosophy of biology about the degree to which the evolutionary process is contingent, or is instead driven by trends that tend to produce convergence on the same solutions. A similar question is, does civilization require a humanoid form? Dolphins, octopi, and crows are intelligent animals, but it's difficult to see how they could build a civilization due to the limits of their morphologies - they simply cannot manipulate tools in the precise ways that humans hands can.

-- A number of authors have suggested that SETI is in some sense unscientific. To take Popper's criterion of falsifiability, for example, it seems that SETI research is organized around a proposition that is unfalsifiable, namely: there exist detectable extraterrestrial civilizations. SETI may therefore provide a good case study for discussion of the demarcation problem of how to distinguish science from non-science.

-- Putting aside the practical problem of the vast distances, is it possible even in principle to communicate with extraterrestrials? Translating human languages, such as Egyptian hieroglyphics, is often hard enough; the difficulties facing any interpreters of extraterrestrial signals will be orders of magnitude greater. Is there anything that could serve as a "universal langauge" - mathematics, for example? (I discuss the problem of extraterrestrial communication in this video.

-- Active SETI is a branch of SETI that attempts to contact extraterrestrials directly by sending messages into space. Various authors have argued that while listening for extraterrestrials is acceptable, we should refrain from sending messages ourselves because it poses potentially catastrhopic risks. We don't know the character or the capabilities of the civilizations we are attempting to contant. For all we know, they may well be able to do us great harm. (I discuss these risks in this video.

-- What will be the impact on our culture of discovering extraterrestrial life? Are contemporary religions such as Christianity compatible with the existence of extraterrestrials?

-- We may well discover simple microbial life elsewhere in the solar system; Mars and Europa, for example, remain possible abodes of life. What moral obligations would we have to such life? What kind of planetary protection policies should we adopt? How can we extend contemporary theories in environmental ethics to cover extraterrestrial life?


Why did philosophers lose interest in extraterrestrials? I don't have the historical competence to answer this definitively, but it's worth noting a few points. By the early 1900s, the scientific justification for belief in extensive extraterrestrial life had collapsed. Closer studies of the other planets of the solar system revealed them to be barren, desolate worlds, wholly unsuitable for life. The "canals of Mars" controversy had brought disrepute to the study of extraterrestrials. Furthermore, by this time the dominant view of the origin of the solar system was the Chamberlin-Moulton encounter hypothesis, which proposed that the solar system formed when another star passed close to the Sun and gravitational interaction drew out huge filaments of material that coalesced to form the planets. Planets were therefore likely to be extremely rare. Whereas the Copernican revolution had initially seemed to provide a scientific basis for optimism, extraterrestrials now once again entered the realm of mere speculation.

At around the same time, philosophy became dominated by the logical positivists, who held that philosophy cannot make empirical claims and instead focused on technical problems of meaning of scientific terms, theory reduction, the nature of scientific confirmation, etc. They saw little value in philosophical speculation on grand topics such as extraterrestrials. In later decades, into the 1940s and beyond, outlandish reports of UFOs and alien abductions have sullied the topic of extraterrestrials still further in the eyes of serious philosophers.

Whatever the reasons for the absence of philosophical study of extraterrestrials, I hope, in light of the great scientific progress on the problem and the wide range of philosophical issues it connects with, that more philosophers turn their attention to it in the near future.

Entity realism and experimental failure

Entity realism, defended most famously by Ian Hacking, is the view that we should be sceptical of scientific theories, but we should believe in unobservable entities that scientists can manipulate and use as tools to study other phenomena. Nobody will ever observe an electron, but we should believe in electrons because we can spray them onto deuterium to help study weak neutral currents. As Hacking says, "When we use entities as tools, as instruments of inquiry, we are entitled to regard them as real."

Discussions of entity realism often assume that experimental or manipulative success is the entity realist's criterion for belief in unobservable entities; Gelfert, for example, says: "In its original form due to Ian Hacking, entity realism postulates a criterion of manipulative success which replaces explanatory virtue as the criterion of justified scientific belief." I suggest that the entity realist would be better off dropping this criterion. Experimental failure often provides just as strong reason for belief as experimental success.

Consider the following case from solar astronomy. Thermonuclear fusion in the core of the Sun occurs by the proton-proton chain, where four protons are converted into a helium nucleus, two positrons, and two electron neutrinos:

4p → ⁴He + 2e⁺ + 2ν_e

Neutrinos interact with matter extremely rarely; a piece of lead one light-year thick would stop only 50% of any neutrinos passing through it. The vast majority of neutrinos produced in thermonuclear fusion pass straight through the Sun unimpeded. So studying neutrino flux gives us a direct view into the core of the Sun. The challenge is how to detect the flux of these particles that interact so weakly with matter.

The first experiment to detect solar neutrinos was developed in the mid-60s by Ray Davis using a tank of perchloroethylene, C2Cl4. Very rarely, a neutrino collided with a chlorine nucleus, converting it into radioactive argon by converting a neutron into a proton. The radioactive argon was then captured by bubbling helium through the tank. Davis used the rate of argon production to measure the neutrino flux. Unfortunately, the Davis experiment was a failure; he detected only 1/4 of predicted neutrino flux (figures are from Longair's The Cosmic Century, p.180):

Predicted flux: 7.9 +/- 2.6 solar neutrino units
Observed flux: 2.1 +/- 0.9 solar neutrino units

What went wrong? Different reactions in the p-p chain produce electron neutrinos of different energies. It was proposed that Davis was detecting only the high-energy neutrinos. Further experiments, such as GALLEX and SAGE, which were based on neutrino collisions converting gallium into radioactive germanium, were designed to detect the low-energy neutrinos. These experiments also failed; they detected only about half of the predicted low-energy neutrino flux. Another suggestion was that something was wrong with the solar models – perhaps we were mistaken about the temperature in the core of the Sun, for example – however, by the 90s experiments in helioseismology were providing robust confirmation of these models, and it became clear that the problem lay with our understanding of the nuclear physics relating to neutrinos (see Longair p.182).

The problem was finally resolved in the early 2000s by the discovery of neutrino oscillation: some of the electron neutrinos produced in the p-p chain spontaneously converted into tau and muon neutrinos, which were not detected by any of the original experiments. However, what is interesting in the context of entity realism is the situation in the decades prior to this. From the mid-60s to the early-00s, across numerous different experiments, predictions failed and experiments did not work as intended. So any argument from experimental success could not apply to neutrinos. Yet it remains the case that scientists used neutrinos as tools, as "instruments of inquiry", in their investigations of the core of the Sun, which seems to be what Hacking requires to justify belief in neutrinos. Indeed, few, if any, scientists working on solar physics doubted that neutrinos were being detected.

This case suggests that there is nothing especially important about experimental success. By analogy, we use hammers to drive nails into walls. There are many ways we might fail: we might smash a hole in the wall, or we might be using a dud nail that snaps when we hit it. None of this would lead us to doubt the existence of the hammer. We don't infer the hammer's existence from its success at driving nails into walls; the fact that we can use and manipulate the hammer in certain ways is enough.

One reason why it's worth exploring cases of experimental failure is that these may provide the entity realist with the resources to answer the persistent objection that entity realism collapses into standard scientific realism. Believing in some entity X, so the objection goes, requires believing theories about X. As Alan Musgrave puts it:

To believe in an entity, while believing nothing else about that entity, is to believe nothing or next to nothing. I tell you that I believe in hobgoblins. "So", you say, "You think there are little people who creep into houses at night and do the housework." To which I reply that I do not believe that, or anything else about what hobgoblins do or what they are like - I just believe in them.

Hacking, as is well known, has claimed that while we shouldn't believe scientific theories about e.g. electrons, there are basic "home truths" about electrons, a basic "common lore" about electrons, that we should all accept. Such home truths include that electrons have negative charge, that they "orbit" (in some sense) atomic nuclei, that they have a rest mass of about 9.1x10-31kg, that certain materials transmit electrons more readily than others, etc. This is enough to give us a substantial belief in electrons, and to allow us to use electrons in scientific experiments.

Theories change but home truths remain. This is illustrated very nicely by the neutrino experiments. Since the neutrino detection experiments failed to confirm the theoretical predictions, it was clear that some part of the theory was wrong. Initially this was attributed to inaccuracies in the solar models, but by the 1990s most scientists turned their attention to the nuclear physics. Neutrino theory was wrong. Arguably then, belief in neutrinos could not be justified by appealing to this theory. Nevertheless, scientists retained certain low-level assumptions or "home truths" about the causal powers of neutrinos, such as that neutrinos are produced by reactions in the p-p chain, and that they can very occasionally collide with chlorine converting it into argon. Perhaps entity realists should turn their attention to examples of experimental failure.

An interesting follow-up question for the entity realist would be, how do we distinguish cases of failed prediction where we nevertheless assume that the experimenters were using a certain unobservable, as occurred in neutrino astronomy, from those cases where we think that nothing important was detected? Joseph Weber's research into gravitational waves might be an example of the latter case.