The A-theory of time holds that there is an objective distinction between past, present and future (or past and future, or past and non-past, or non-future and future, or present and past, vel caetera). A standard argument against the A-theory of time is that scientific theories do not make this distinction, and hence the distinction is bogus. This is a bad argument (for a true conclusion!). Here are some reasons why.
First, as has been noted before, it is simply false that scientific theories do in fact fail to make this distinction. It seems like a perfectly fine scientific theory to say that birds "arose nearly 200 million years ago". But of course this claim presupposes a distinction between past, present and future, since otherwise the "ago" makes no sense.
Maybe this objection is uncharitable. Perhaps the correct claim is that no fundamental scientific theory presupposes a distinction between past, present and future. A worry I have about this version of the argument is the question of the criteria for fundamentality—there may be a circularity in that we reject the fundamentality of a theory that does make a distinction between past, present and future.
Perhaps, then, we should say:
- The laws of nature, as we know them, make no reference to the objective distinction between past, present and future, but treat all times on par.
- If the laws of nature make no reference to some distinction, then that distinction is probably not objective
- If the laws of nature make no reference to some distinction, and the distinction concerns a topic amenable to scientific examination, then the distinction is likely not objective.
If we can know a priori that science must come to a particular conclusion, then our argument for the conclusion is no longer a scientific one. For the peculiar epistemic authority of science is based on scientific methods' responsiveness to empirical data, and in such a case we have no responsiveness. Given an a priori argument that science must come to a particular conclusion, we might accept the conclusion, but if we do so, we do so on the basis of a meta-scientific argument, not a scientific one.
Now, I think we can know a priori that the laws of nature that science arrives at will not have an objective distinction between past, present and future. For the laws of nature that science formulates are timeless. Consider what a law that makes such a distinction would be like. It might, for instance, say that some property is had by an entity now but not in the future. But then the sentence will shortly no longer be true. And that is not the sort of sentence that formulates anything we recognize as a scientific law.
But if we know a priori that the laws of nature that science arrives at will not make a distinction between past, present and future, then we cannot use the lack of such a distinction as a scientific argument against the A-theory.
Of course there may be other and maybe even scientific arguments against the A-theory.