In describing the realm of spirit as _ex hypothesi_ known, that of Nature unknown, to the alchemists, I have made one important omission, and that, if I may use the name of a science to denominate a complex of crude facts, is the realm of physiology, which, falling within that of Nature, must yet be classed as _ex hypothesi_ known. But to elucidate this point some further considerations are necessary touching the general nature of knowledge. Now, facts may be roughly classed, according to their obviousness and frequency of occurrence, into four groups. There are, first of all, facts which are so obvious, to put it paradoxically, that they escape notice; and these facts are the commonest and most frequent in their occurrence. I think it is Mr CHESTERTON who has said that, looking at a forest one cannot see the trees because of the forest; and, in _The Innocence of Father Brown_, he has a good story ("The Invisible Man") illustrating the point, in which a man renders himself invisible by dressing up in a postman's uniform. At any rate, we know that when a phenomenon becomes persistent it tends to escape observation; thus, continuous motion can only be appreciated with reference to a stationary body, and a noise, continually repeated, becomes at last inaudible. The tendency of often-repeated actions to become habitual, and at last automatic, that is to say, carried out without consciousness, is a closely related phenomenon. We can understand, therefore, why a knowledge of the existence of the atmosphere, as distinct from the wind, came late in the history of primitive man, as, also, many other curious gaps in his knowledge. In the second group we may put those facts which are common, that is, of frequent occurrence, and are classed as obvious. Such facts are accepted at face-value by the primitive mind, and are used as the basis of explanation of facts in the two remaining groups, namely, those facts which, though common, are apt to escape the attention owing to their inconspicuousness, and those which are of infrequent occurrence. When the mind takes the trouble to observe a fact of the third group, or is confronted by one of the fourth, it feels a sense of surprise. Such facts wear an air of strangeness, and the mind can only rest satisfied when it has shown them to itself as in some way cases of the second group of facts, or, at least, brought them into relation therewith. That is what the mind--at least the primitive mind--means by "explanation". "It is obvious," we say, commencing an argument, thereby proclaiming our intention to bring that which is at first in the category of the not-obvious, into the category of the obvious. It remains for a more sceptical type of mind--a later product of human evolution--to question obvious facts, to explain them, either, as in science, by establishing deeper and more far-reaching correlations between phenomena, or in philosophy, by seeking for the source and purpose of such facts, or,