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?
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