case such a criterion is theoretically worthless. The whole affair is a matter of degree, and we cannot draw any sharp line between perception and inference. As soon as this is realised, our difficulties are seen to be purely verbal and therefore unimportant. It will be observed that we are not attempting at present to say what constitutes perception, but only what kind of behaviour on the part of a person whom we are observing will justify us in saying that he has perceived this or that feature of his environment. I suggest that we are justified in saying that a man “perceives” such a feature if, throughout some such period as a day, there is some bodily act which he performs whenever that feature is present, but not at any other time. This condition is clearly sufficient, but not necessary--that is to say, there may be perception even when it is not fulfilled. A man’s reaction may change through conditioning, even in so short a period as a day. Again, there may be a reaction, but one which is too slight to be observable; in this case the criterion of perception is theoretically satisfied, but not practically, since no one can know that it is. We often have evidence later on that something was perceived, although at the moment there was no discoverable reaction. I have frequently known children repeat afterwards some remark which, at the time, they seemed not to have heard. This sort of case affords another kind of evidence of perception, namely, the evidence afforded by a delayed response. Some people will sit silent and impassive in a company of talkers, giving no evidence that they are listening; yet they may go home and write down the conversation verbatim in their journals. These are the typical writers of memoirs. More remarkable still, I know one man--a man of genius, it is true--who talks incessantly, who yet, after meeting a total stranger, knows exactly what the stranger would have said if he had been given the chance. How this is managed, I do not know; but such a man is rightly called “perceptive”.