world, not a detail; one that concerns the structure of the world, a valid law, not a single unique fact. This distinction may be described as the difference between the real nature of the Universe and the accidental form in which this nature manifests itself.
Correspondingly, the reasons why a given problem is insoluble may be of two entirely different kinds. In the first place, the impossibility of answering a given question may be an impossibility in principle or, as we shall call it, a logical impossibility. In the second place, it may be due to accidental circumstances which do not affect the general laws, and in this case we shall speak of an empirical impossibility.
In the simple instances given above, it is clear that the impossibility of answering these questions is of the empirical kind. It is merely a matter •of chance that neither Plato nor any of his friends took exact notes of his doings on his fiftieth birthday (or that such notes were lost if any were taken); and a similar remark applies to the questions concerning the weight of Homer and things on the other side of the moon. It is practically or technically impossible for human beings to reach the moon and go around it, and most probably such an exploration of our earth's satellite will never take place. But we cannot declare it impossbile in principle. The moon happens to be very far off; it happens to turn always the same side towards the earth; it happens to possess no atmosphere in which human beings could breathe — •but we can very easily imagine all these circumstances to be different. We are prevented from visiting the moon only by brute facts, by an unfortunate state of affairs, not by any principle by which certain things were deliberately withheld from our knowledge. Even if the impossibility of solving a certain question is due to a Law of Nature, we shall have to say that it is only empirical, not logical, provided we can indicate how the law would have to be changed in order to make the question answerable. After all, the existence of any Law of Nature must be considered as an empirical fact which might just as well be different. The scientist's whole interest is concentrated On the particular Laws of Nature; but the philosopher's general point of view must be independent of the validity of any particular one of them.
It is one of the most important contentions of the philosophy I am advocating that there are many questions which it is empirically impossible
