THE scientific analysis of psychical phenomena must, I believe, aim to explain these phenomena according to laws of physical chemistry. I know very well that many people would hold that even a complete physico-chemical analysis of all psychic phenomena must still leave the "truly psychical" unexplained. I do not agree with such an opinion, but as we are to-day still very far from the ideal of a complete physico-chemical analysis of psychical phenomena, there is nothing to be gained by quarrelling about just how much scientific illumination and satisfaction we shall attain when that goal is once reached. On the second point, however, a general agreement may be reached, namely, first, that we must undertake and carry out a physico-chemical analysis of psychical phenomena; and, second, that for such an analysis the same principles of investigation are required as for the physicochemical analysis of the very much simpler processes in inanimate nature.

Twenty-two years ago I came to the conclusion that what we call "will" in many lower animals is nothing but the phenomena of tropisms well known in plants, especially through the work of Sachs. In a series of articles, of which the first two appeared^[2] in January, 1888, I have tried to establish this view, and I will now summarize the facts briefly, and try to do away with some of the difficulties which zoologists and psychologists have experienced in applying my theories. To my mind the essential value of my theory lies in the preparation