the psychology of sensation. These are the two poles, and they stand widely asunder, of the psychological world; the, doctrine of innate ideas on the one hand, and the doctrine of sensation on the other hand. You will understand how widely apart these doctrines are placed if you will bear in mind the extremes which I have stated, extremes which they approach if they do not exactly reach. The extreme doctrine of innate ideas allows nothing to foreign sources, but finds the origin of all cognition in the mind itself; the extreme doctrine of sensation allows nothing to the mind itself, but finds the origin of all cognition in foreign sources. That antithesis may enable you to keep in mind and to understand generally the character and tendency of the two great psychological schemes which I say have divided the philosophical world. It may here occur to you that a third alternative is possible as a solution of the problem respecting the origin of our knowledge, and that this third solution is the truest and most natural of any. Why, you will ask, why may we not combine into one the two solutions just given, and thus obtain the most correct and the most tenable explanation? Why may we not say that our knowledge is due neither entirely to the mind itself, nor entirely to the action of external things, but that it is the joint result of both these constituents? Now there can be no doubt that the true answer to the problem does lie somewhere in this middle alternative. But there is a difficulty in ad-
