The fact of this correlation has been established by two principal lines of evidence. In the first place, we find all through the animal kingdom that _size of brain and complexity of nervous system are matched by range and complexity of mental phenomena_. The brain of man is, by absolute measurement, an organ of great size; it is heavier than that of any other animal with the exception of a few of the very largest (such as the elephant); and in these cases the superior weight is due, not to superior development of the elaborating part of the brain, but to the bulk of the receiving and emitting portions, which are of a size to correspond with the bulk of the body. The brain of man is also relatively, as compared with the weight of the whole body, heavier than the brain of any other animal with the exception of a few of the most highly developed small mammals (such as certain monkeys); and in these cases again the superiority depends on the bulk of the receiving and emitting portions of the brain, which reflect the keen sensitivity and muscular agility of the animal. We know, on the other side, that the mental life of man is richer than that of any other creature. Secondly, we find that _disturbance of certain parts of the brain indicates a certain form of mental disturbance_; and, conversely, that particular forms of mental disturbance indicate disturbance of particular parts of the brain. One may become blind from injury to the brain as well as from such defect of the eye as prevents optical impressions from reaching the brain. These are the two lines of evidence. How, though, you may now ask, do we know anything about the distribution of mental phenomena in the animal kingdom? =How do we know that the lower animals live in mental worlds?= and still more how can we say anything as to the nature of the phenomena that make up those worlds? Consider first _the case of your fellow-men_. You do not doubt that they have experiences like your own; you take them for granted, accept them instinctively as your kin, and are able—the better as you know them better—to put yourself in their place. If, however, you had to argue the matter with a sceptic, you would point to the facts of our common life. Man’s family life, social life, civic life, national life, is based on the assumption that human experience is alike for everyone, and would be impossible if the assumption were falsified by the facts. All these forms of life, for instance, presuppose language and laws; and language and laws necessarily imply a community of experience. You would point, also, to likeness of physical organisation, likeness of sense-organs and nervous system; and you would point, lastly, to conduct or behaviour. When you feel in a certain way, you act in a certain way; your behaviour expresses your feeling; and when, under the same circumstances, a creature of like organisation regularly acts in the same way, you have a right to infer that this creature has a like feeling.