events into one universe, in spite of their many important differences. The exact details of this must be left to the symbolic logician; but the complexities which arise even in the simple example of family relationships will show the reader that the complication of Nature as a whole is compatible with the ultimate relations between its elements being comparatively few and simple. The mistake is to try to force Nature as a whole into the mould which fits one important part of it; and then to suppose that, because this attempt breaks down, Nature as a whole has no structure at all, but falls into completely isolated and incoherent fragments. There are, I believe, two different levels of "simplicity," and between them there is a region of "complexity." There is the lower kind of simplicity, which we find when we isolate one fragment of Nature from the rest, and ignore all the awkward facts that refuse to fit into the scheme which applies to this fragment. There is, or there well may be, a higher kind of simplicity, where we have recognised the fundamental structure of Nature as a whole, and have seen how the structure of special regions of Nature is just a special case of these fundamental relations. But, in order to pass from the lower to the higher kind of simplicity, we must traverse an intermediate stage of confusion and complexity, in which we confront the lower simplicity with all the awkward facts which it has ignored. This is a task in which we can all help, if we keep our heads clear and refuse to be put off with cheap and easy explanations. The final stage, that of finding the simple plan on which all this complexity is constructed, can only be accomplished by men who combine the insight of genius with technical mathematical ability of the highest order. To this combination of gifts few of us can lay claim, and the present writer is certainly not one of those who can. In our day one man, Einstein, has shown what such a combination can accomplish within the region of physics. We still await the man who

will show us in detail how the world of physics and the world of sensible appearance are united into the one whole of Nature. The utmost that we can claim to have done here is to have stated some of the facts which he will have to take into account and to unify.

The following additional works may be consulted with advantage :

A. N. WHITEHEAD, The Principles of Natural Knowledge, Parts
II. and IV.

The Concept of Nature, Chaps. I., II.

and VII.

The Principle of Relativity, Chaps. II. and

IV.

B. A. W. RUSSELL, Our Knowledge of the External World,
Lects. III. and IV.

The Analysis of Mind, Lects. V. and VII.

S. ALEXANDER, Space, Time, and Deity, Bk. III.

H. BERGSON, Matter and Memory.

G. E. MOORE, Philosophical Studies.

G. F. STOUT, Mind, Vol. XXXI. No. 124.