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.