Let us now repeat Einstein's assertion "that the laws of Nature, the processes of Nature, exhibit a much higher degree of uniformity of connexion than is contained in our time-causality! It is possible that what belongs to a definite cross-section of time may in itself be entirely devoid of structure, that is, it might contain everything that is physically conceivable, even such things as, in our ordinary physical thought, we consider impossible of realization, for example, iron of any arbitrary specific gravity." It seems to me that the non-physicist will, perhaps, gain a clearer insight into these highly significant words of Einstein, now that he has received the assistance of these physiological considerations. It must be granted that the philosophic grounds of Einstein are quite different and lie much deeper than those of von Baer, who starts from organic functions and ends by arriving at a mysterious relativity that is yet consistent in itself. Nevertheless, there is one point of contact, inasmuch as in each case possibilities that lie apparently _extra naturam_ are suggested. Einstein says: "Hitherto we have regarded physical laws only from the point of view of _causality_, inasmuch as we always start from a condition known at a definite cross-section of time, as, for example, a section corresponding to the present moment." At our own risk an easy paraphrase of his words will be attempted: The time-section of the present contains for us the sum of all previous experiences, out of which the necessary course of our thought sifts out the category of causality. What is not present in experience cannot appear in our causality. Let us consider for a moment Hume's example of the Indian who has never known ice. Without being told, and if he is dependent only on his own sensations, he would never learn that water freezes in cold climates. The influence of cold on water is not gradual, corresponding to an increase of cold, and not one that may be anticipated in all its consequences, but at the freezing-point water, which a moment before was a very mobile liquid, passes into a very rigid solid. The causality of the Indian cannot account for this. If we tell him of this phenomenon, he has two courses open to him. Either he refuses to believe it--and this would be quite natural, since rigid water is to him as meaningless as is a square circle to us. Or else he believes the story, and then his list of categories incurs a break, passing through the middle of causality. He has then to reconcile himself to the assumption that something that is meaningless to him and that stands outside the connexion of cause and effect is possible of realization. Up to that moment, in his time-section of the present, there was no room for it in his causality. To Torricelli the conception of liquid air, which we have been able to prepare only since 1883, would have appeared impossible and incompatible with his causality.