It is barely fifty years since the problem of supreme interest to mankind--the problem of the nature, possibilities, and destiny of man--began to be studied in a really scientific way; yet in that half century more progress has been made toward its solution than in all the previous thousands of years that have elapsed since man first asked himself: What am I? What are my capabilities? Shall I be, after I have ceased to exist here on earth? Armed with instruments of the most delicate precision, devising novel methods for exploring the body and the mind in their mutual ramifications, modern investigators have thrown a flood of new and largely unexpected light on the great questions at issue, and have opened vistas of hope and aspiration and actual achievement undreamed of by the vanished peoples of bygone times. At first sight, to be sure, much of their effort appears to be irreparably, even wantonly, destructive, and perhaps nowhere more so than in the blows they have dealt at the traditional conception of the central fact in man's psychical make-up--that intangible entity variously known as the ego, the self, the personality, animated and governed by an indwelling, unifying principle, the soul. Every man instinctively believes that there is only one of him. He feels that, no matter how his thoughts, his sensations, his emotions may change in the course of time, he himself will remain essentially and permanently the same. Putting this belief into metaphysical language, he declares, with the excellent Thomas Reid: "The conviction which every man has of his identity ... needs no aid of philosophy to strengthen it; and no philosophy can weaken it without first producing some degree of insanity.... The identity of a person is a perfect identity; wherever it is real it admits of no degrees; and it is impossible that a person should be in part the same and in part different, because a person is a monad, and is not divisible into parts."[45] [45] Thomas Reid's "Essay on the Intellectual Powers of Man," pp. 228-231 (James Walker's edition of 1850). But the modern explorer of the nature of man, replies: "You are wrong, my friend. Your self is very far from being the simple, stable unity that you imagine it to be. In reality it is most complex and most unstable, easily breaking up, and sometimes breaking up so completely that it may even be replaced by an entirely new self. You do not believe this? I can prove it to you from the facts not only of scientific experiment, but also of everyday observation." Naturally, in support of this statement, stress would be laid on instances resembling the strange case of BCA, just narrated. And although cases at all similar to the BCA affair are extremely uncommon there are a number on record evidencing in other ways so-called "total dissociation of personality." For example: