temptation to exaggerate the mental endowments of the lowest creatures, to make their minds miniature copies of the human. Romanes' books, for instance, show over and over again how a psychologist, working in the interests of mental evolution, may overestimate the range and complexity of the animal consciousness. Still, this is the one path that psychology can follow. The biologist, on the other hand, thinks his world, when he thinks consistently, in terms of physics and chemistry. He is also accustomed to think from below upwards. His natural tendency, then, is to carry physical and chemical principles as high in the scale of life as they will go. He has his inconsequences, as the psychologist has his exaggerations; and his besetting inconsequence is to admit the presence of mind in animals higher than those which he has himself examined. But science is not inconsistent because her representatives may sometimes nod. It is a postulate of mechanistic biology that physical and chemical principles will carry the biological student all the way, from the algæ to man. Consciousness does not fall within his horizon. Until, then, the interactionist has converted his biological colleagues to vitalism, and thence to the admission of mental process as an equivalent of physical energy, we must conclude that the two fields of enquiry, the psychological and the biological, do not overlap. There is no reason why the biologist, granted a steady increase of natural knowledge, should not some day reduce all the movements, say, of the monkeys, to physico-chemical terms, to a system of simple or complex 'reflexes.' That is what he is on the way to do. On the other hand, the reduction of all the movements of Paramecium to a single 'reflex' type does not prove to the psychologist that the creature has not (or has not had) a mind. Biology and psychology, if I may change the metaphor, meet and pass on a double track. They do not collide, but neither do they turn out to be two halves of a single train of thought.
We may ask, secondly, for a clearer understanding of what has been called the 'objective criterion of mind.' The phrase is open to the objection that it contains a contradictio in adjectivo; how can there be an objective criterion of the subjective? But we may waive this, and assume that an empirical correlation is possible. Let us suppose, then, that biology and psychology agree to ask the question: How are we to tell, by watching a lower animal's movements, whether or not it has a mind? And let us suppose, further, that they are agreed upon their answer. The answer must be of this kind: If you see so and so, then you may infer the presence of consciousness. Beyond that, no answer has gone, and no answer can possibly go. Because this animal does this thing, it has a mind; this other animal does not do this thing, therefore—what? Therefore, we do not know whether it has a mind. It may not, of course, but then again, it may. The biologist, 1 repeat, may