Can there ever be a meaningful distinction in science between the "unknown" and the "unknowable"? I see no reason why science should not,in 100,000 years or so, unlock what now seem to be unknowable questions like the nature of a Prime Mover, if he exists, simply by accruing more and more knowledge of the universe. We know pretty much what happened a trillionth of a second after the Big Bang and we acquired this knowledge in about 100 years so why assume everything before that is unknowable? Surely the scientific method would insist that this is "presently unknown". Is it that metaphysics and the persistence of religious belief color our approach? Is "unknowable" even a valid term in philosophy, and, if so, what definitive, unassailable examples are there of it (which would also apply, say, 100,000 years from now)? Thanks in advance.

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Let's begin by distinguishing two senses in which something might be said to be unknowable. In some cases something is said to be unknowable because it isn't the kind of thing about which, in principle, knowledge could be had. For example, do you know the minimum number of hairs a man must have on his head to escape being bald? Well, there is no such minimum number to be known or discovered, because the concept of baldness is too vague for this. Do you know what time of day it is now on the sun (Wittgenstein)? Would you be happier dead? Again, there's nothing to be known or discovered in these regards. Philosophers have said about such cases that "there is no fact of the matter." Let us set these cases aside, because they are not the ones that interest you.

In the cases that interest you, there is a fact of the matter. And the claim is then either that it is impossible for us (human beings including all future generations) to know this fact or even, more dramatically, that it is in principle impossible for any beings to know this fact.

I believe that, contrary to what you are suggesting, there can be and there really are unknowables of both these kinds. Let us begin with the first kind of case, with knowledge others might have but we humans can never acquire.

Physicist believe that the universe is expanding and that some parts of it are moving away from us (or we from them, this does not matter) at extremely high speeds. According to their theory, it is physically impossible for us ever to catch up with those objects so as to examine aspects of them that leave no trace in their emissions. Intelligences living in those parts (if there are such) can know those details, but human beings cannot ever possibly find them out.

You may object that perhaps our physicists are wrong. Perhaps future scientists will invent a form of space travel that is much faster than the speed of light and would allow human beings to travel to those parts and examine them closely. This objection is sound, but it misses its mark. It shows that what we now believe to be unknowable may yet turn out to be knowable. The objection does not show that there could not be anything unknowable. After all, present physics might also turn out to be right on this point, in which case human beings can really never possibly know various details about those fast receding parts of the universe.

Now let's proceed to unknowables of the other kind, facts that no one could possibly ever know. Some mathematical and perhaps also some physical facts are so complex that there isn't enough stuff in the universe to represent or to encode them; it is in principle impossible to know the first 10^1000 prime numbers, for example. Some experiences are unattainable for any physically possible conscious life form: No one can know what it is like to be at the center of the sun, because there is simply too much heat and pressure to introduce and maintain there the complex molecules necessary for conscious life. No one can know the last thoughts of beings whose planet was sucked into a black hole (whose gravitational pull, so physicists tell us, no information-bearing emissions can overcome). No one can know what laws of physics held true in the universe a billion years before beings like oneself came into existence (I'm a bit skeptical here about the knowledge you claim about the moment of the Big Bang -- knowledge that relies on extrapolating present natural laws backward in time). No one can know through which of two little holes some minimal particle or photon reached a sensor, because any way of acquiring this information would have interfered with its reaching the sensor.

Again, some of these examples are disputable. Perhaps it will turn out that some of these things can be known after all. But your question, as I understood it, was whether there might be unknowables in the second sense. And to this question the answer is yes. There might well be. And the progress of science may well teach us that things we deemed knowable are in fact unknowable (just as it may indeed also teach us that things we deemed unknowable are knowable).

This leaves us with a thought close to Socrates. A good part of intellectual progress consists in knowing what we can and what we cannot know. Or in the words of modern philosopher Clint Eastwood: "A man's got to know his limitations." (And women, too.)

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