as far as I am aware, at what hour of the day Socrates was born and it is quite likely that this will never be found out by any one. So there certainly are limits to human knowledge.
But you notice immediately that such limits as I have just mentioned, are of a different kind from those which play the important role in the systems of philosophy. They are — as it were, less serious and do not interest the philosopher, although he must admit their existence.
It is necessary, in fact, to make a very important distinction here. There are two entirely different kinds of impossibility of knowing: a logical one, and an accidental or factual impossibility. In Kant's philosophy, for instance, it is logically and absolutely impossible for any human understanding to acquire knowledge of "things in themselves" — this means, it is unthinkable, we cannot describe what should have to be done in order to get such metaphysical knowledge, and we cannot imagine any beings that could be capable of it (although Kant believed he could describe such beings by saying they would have to be endowed with "intellectual intuition" — which is a contradiction, for intellect — if I may use our own terminology — has to do with form, and intuition with content). In this case knowledge of things in themselves would be impossible in principle. In the case of the far side of the moon, or the birth hour of Socrates, however, the impossibility of knowing them, has quite a different character.
It is not due to a principle, but to accidental circumstances, it is of a practical or technical, not of a logical nature. We know exactly what would have to happen if we wanted to know about the birth of Socrates: we would have to find some old papyrus or inscription in which a reliable and accurate account of the event was given, and it is just an unfortunate chance that such a document, as far as we know, does not exist. But it might exist; we can easily imagine it, and that means that our knowledge of the fact just happens to be impossible, the impossibility is a consequence of accidental circumstances, not of the nature of knowledge itself. Similarly there is no difficulty in describing the circumstances which would enable us to know the back part of the moon: we should simply devise some means of going around the moon and look at it. This happens to be technically impossible at present, but may not be so in the future; and even if we were certain that human beings will never succeed in going around the moon,
