Kant maintained that, in virtue of our mental constitution, we deal with the raw material of sense-impressions by means of certain “categories” and by arranging it in space and time. Both the categories and the space-time arrangement are supplied by us, and do not belong to the world except as known by us. But since our mental constitution is a constant datum, all phenomena _as known_ will be spatio-temporal and will conform to the categories. Among the latter “cause” is the most important. Thus although there may be no cause in the world as it is in itself (a point on which Kant was inconsistent in the interest of morals), yet phenomena, _i.e._ things as they seem to us, will always have other phenomena as their causes. And although there is no time in the real world, things as they appear to us will be some earlier and some later. Space, again is supplied by us, and therefore geometry can be known _a priori_, without our having to study the outer world. Kant thought that Euclidean geometry was quite certainly true, although it could not be proved by logic alone, since Euclid’s axioms could be denied without self-contradiction. It was on this question of geometry that the weakness of Kant’s system first became obvious. It was found that we have no grounds for regarding Euclidean geometry as quite true. Since Einstein, we have positive grounds for regarding it as not quite true. It appears that geometry is just as empirical as geography. We depend upon observation if we want to know whether the sum of the angles of a triangle is two right angles just as much as if we want to know how much land there is in the western hemisphere. With regard to the “categories” there are equally great difficulties. Let us take “cause” as our illustration. We see lightning, and then we hear thunder; as phenomena, our seeing and hearing are connected as cause and effect. But we must not--if we are to take the subjectivity of “cause” seriously--suppose that our seeing or our hearing has an outside cause. In that case, we have no reason to suppose that there is anything outside ourselves. Nay, more: what _really_ happens when we see is not, according to Kant, what we perceive by introspection; what really happens is something without a date, without a position in space, without causes and without effects. Thus we do not know ourselves any better than we know the outside world. Space and time and the categories interpose a mirage of illusion which cannot be penetrated at any point. As an answer to Hume’s scepticism, this seems a somewhat unsuccessful effort. And Kant himself, later, in the _Critique of Practical Reason_, demolished much of his own edifice, because he thought that ethics at least must have validity in the “real” world. This part of his philosophy, however, is usually ignored by his followers or apologetically minimised.