When we turn from the genetic study of the soul to the great question of its immortality, we come to that highest point of superstition which is regarded as the impregnable citadel of all mystical and dualistic notions. For in this crucial question, more than in any other problem, philosophic thought is complicated by the selfish interest of the human personality, who is determined to have a guarantee of his existence beyond the grave at any price. This "higher necessity of feeling" is so powerful that it sweeps aside all the logical arguments of critical reason. Consciously or unconsciously, most men are influenced in all their general views, and, therefore, in their theory of life, by the dogma of personal immortality; and to this theoretical error must be added practical consequences of the most far-reaching character. It is our task, therefore, to submit every aspect of this important dogma to a critical examination, and to prove its untenability in the light of the empirical data of modern biology. In order to have a short and convenient expression for the two opposed opinions on the question, we shall call the belief in man's personal immortality "athanatism" (from _athanes_ or _athanatos_ == immortal). On the other hand, we give the name of "thanatism" (from _thanatos_ == death) to the opinion which holds that at a man's death not only all the other physiological functions are arrested, but his "soul" also disappears--that is, that sum of cerebral functions which psychic dualism regards as a peculiar entity, independent of the other vital processes in the living body. In approaching this physiological problem of death we must point out the _individual_ character of this organic phenomenon. By death we understand simply the definitive cessation of the vital activity of the _individual_ organism, no matter to which category or stage of individuality the organism in question belongs. Man is dead when his own personality ceases to exist, whether he has left offspring that they may continue to propagate for many generations or not. In a certain sense we often say that the minds of great men (in a dynasty of eminent rulers, for instance, or a family of talented artists) live for many generations; and in the same way we speak of the "soul" of a noble woman living in her children and children's children. But in these cases we are dealing with intricate phenomena of _heredity_, in which a microscopic cell (the sperm-cell of the father or the egg-cell of the mother) transmits certain features to offspring. The particular personalities who produce those sexual cells in thousands are mortal beings, and at their death their personal psychic activity is extinguished like every other physiological function.