Consider the following toy example of a law of nature: All electrons are charged. On an eternalist theory of time, the following expression correctly captures the logical structure of this law.
- ∀x(Ex→Cx) & ∀t[P(t)→Wast(∀x(Ex→Cx))] & ∀t[F(t)→Willt(∀x(Ex→Cx))],
This should be at least a little embarrassing to the presentist. That all electrons are charged seems to be is a very simple law. But (2) is far from simple. Moreover, the analyses may sometimes be even more complex. Consider a putative fairly simple law that makes reference to two different times, say that
- exposure to V tends to cause a disease D.
- exposure to V will tend to cause D, and past cases of exposure to V tended to have caused, be causing or be about to be causing D, and future cases of exposure to V will tend to cause D.
But the concern is not merely esthetic (though I do think beauty is a guide to truth). Suppose that our evidence is equally well explained by two general claims, one of which is both (a) significantly simpler and (b) significantly logically weaker. Then we should not have much confidence in the explanation that is both more complex and stronger. For instance, suppose that all observed ravens are black. This is equally well explained by two general claims: That all ravens are black and that all ravens and geese are black. The claim that all ravens are black is both significantly simpler and significally weaker. We should not go for the more complex explanation that all ravens and geese are black. (If we have no other evidence, though, we might cautiously accept the explanation that all birds are black, because while it's logically stronger, it's no more complex.)
Very well. Now, what is my evidence about electrons? Let's oversimplify the evidence by saying that the evidence is that all the electrons we've observed carefully enough have been charged. This observation can be equally well explained either by (2) or by:
In other words, Presentism combined with a plausible thesis about which explanation one should not accept leads to inductive scepticism.