be no longer impossible for human beings to look at the back of the moon. So we see that our question certainly belongs to the first group; the reasons why it cannot be answered are merely of an accidental nature. This may interest the scientist, but the philosopher is not concerned with it, he is worried by the other group of problems: those which are insoluble in principle: the reason why they cannot be solved is not an accidental state of affairs in the universe, but it seems to lie deeper; in this second case we speak of a philosophical or logical impossibility.
The difference between the two groups is this, that in the first one we can at least imagine means of finding a solution, even if these means exist nowhere in the world, whereas in the case of philosophical impossibility no imagination can bring us nearer to the answer; there are no ways on which even imagination could try to reach the goal. We cannot imagine what we would have to do or what would have to happen in the world in order to lead us to the answer of our former question: “How can a sensation arise from motions of molecules of the brain?” This question, therefore, belongs to the second group; philosophers have always worried about it. In most philosophies there are problems of this kind: certain questions within them are believed to “pass our understanding” or to be mysteries which we cannot fathom.
Our “Philosophy of Experience” takes an entirely different attitude. In order to understand it let us ask: “What is the criterion by which we decide whether an ‘insoluble’ problem belongs to the first or to the second group?” I think the criterion must be stated in this way: All the questions that can in principle be answered (including those that may at any one time or place be technically insoluble) are always answerable in one way, namely by reference to some observation (be it of nature or of ourselves), or by any scientific method which always pre-supposes observation, i. e. the occurrence of some sense impressions — in short, by experience.
A question is in principle answerable (I should like to say: it is a “good question”) if we can imagine the experiences which we would have to have in order to give the answer. An answer to any question is always a proposition. In order to understand a proposition we must be able exactly to indicate those particular circumstances that would make it true and those other particular circumstances that would make it false. “Circum-
