With all respect I venture to say it is not possible. The pretension to such knowledge is at once dangerous and misleading; and though many things in a book of this description may be true and instructive, the great bulk of it must be pure conjecture. So far as my experience goes, no two criminals are alike; but while they vary as ordinary people vary, they are far more careful than ordinary people to conceal their thoughts, and, believe me, they are far more successful in their endeavours. As a matter of fact, the criminal, especially the habitual criminal, lives in a world of self-repression. In prison he is not only shut up, but he also shuts himself up. He gives nothing away to his would-be investigators. He is cute enough to know what the investigator is after, and clever enough to give answers that will please his questioner and confirm him in judgments that he has already formed: though probably there is not more than a grain of truth in the whole of the answers he will give. I am led to these remarks by the fact that all round me as I write are books that deal with the crime of the world, hundreds of them, and a weird collection they form. As I sit and look at them they fascinate me, almost cast a spell over me. But when I rise from my desk, take one into my hand and read but a single page, I am disillusioned. For I realise that the writer knows no more of the true inwardness of things appertaining to the criminal mind, than ordinary observant men may know. True, he can mystify us with scientific terms; he can talk about the conscious and the subconscious, and many other speculative things. He can give us measurements of the body and describe the exact angle at which a criminal’s ears stand, and the angle at which his chin recedes. But when from these he proceeds to reveal the recesses of the mind at the back of the big ears, or the little chins, we feel that we are on an equality with the writer, for we know as much upon the matter as scientists or specialists can know. Now these books have been written by all sorts of people: doctors of law and of medicine, scientists, professors, governors and chaplains of prisons, journalists and self-appointed specialists. Mostly they come to us from the continent of Europe, where the various schools of psychological thought contend for mastery. The Lombrosian school tells us how to detect the criminal by his physical conformation, or by the convolutions of his brain; but as the brain cannot be dissected while the criminal lives, this method of identification is of no practical use. Another school will tell us how to detect the criminal by his behaviour whilst undergoing interrogation or standing his trial, when, not only his appearance, but his actions also are to be closely observed by judge and jury.