Mark asked:

How might it be the case that we could not ask a question, the answer to which could not be known?
And, what kind of question is the above?

Can you suggest any pertinent reading material that might help me with it?

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The question you are asking is a question of metaphysics, which concerns our understanding of the
concept of truth. If a question canbe asked, one of the things which follows from that is that we can
grasp what it would mean for that question to havean answer. Of course, not all questions which
have answers, are questions we can answer. For example, most people would agree that the
question how many times I clicked the mouse today has an answer, but if I am not running a program
on my computer which counts mouse clicks, then barring total recall there is no way I can discover
what that answer is. I can ask the question, but I cannot answer it.

But now consider the question, Has there ever been, in the history of the universe, a period of time
during which absolutely nothing happened?In other words, can there be such a thing as an empty
time? If you allow that there might, sometimes, occur periods of empty time, then it follows that it is
logically possiblethat in between each key tap as I write these words, the universe stops still for a
million years. There is a case for saying that because we cannot conceive of what it would be to
'know' that an empty time has ever occurred, that is one question we cannot even ask.

There is an interesting discussion of this question in Richard Swinburne's book Space and Time.
Swinburne devises a clever thought experiment which shows how we could, indirectly, have evidence
for an empty time. Suppose the universe is divided into three regions, A, B and C. All processes in
region A stop for one year out of every two years. All processes in region B stop for one year out of
every three years. And all processes in region C stop for one year out of every four years. Then a
quick calculation shows that the three periods of total inactivity will coincide every twelfth year.

The only problem is that Swinburne's example cheats, in assuming that the periods during which all
processes in the universe come to a stop are predictable. It does not help to explain how it could
happen that all processes in the universe stopped unpredictably, as in the case of a million years
passing in between each key tap.

Geoffrey Klempner