§ 98. There is similar complexity in the problem concerning _immortality_. Were the extreme claims of naturalism to be established, there would be no ground whatsoever upon which to maintain the immortality of man, mere dust returning unto dust. The philosophical concept of immortality is due to the supposition that the quintessence of the individual's nature is divine.[213:18] But several possibilities are at this point open to us. The first would maintain the survival after death of a recognizable and discrete personality. Another would suppose a preservation after death, through being taken up into the life of God. Still another, the theory commonly maintained on the ground of rationalistic and idealistic metaphysics, would deny that immortality has to do with life after death, and affirm that it signifies the perpetual membership of the human individual in a realm of eternity through the truth or virtue that is in him. But this interpretation evidently leaves open the question of the immortality of that which is distinctive and personal in human nature. [Sidenote: The Natural Science of Psychology. Its Problems and Method.] § 99. So far we have followed the fortunes only of the "spirit" of man. What of that lower soul through which he is identified with the fortunes of his body? When philosophy gradually ceased, in the sixteenth and seventeenth centuries, to be "the handmaid of religion," there arose a renewed interest in that part of human nature lying between the strictly physiological functions, on the one hand, and thought and will on the other. Descartes and Spinoza analyzed what they called the "passions," meaning such states of mind as are conditioned by a concern for the interests of the body. At a later period, certain English philosophers, following Locke, traced the dependence of ideas upon the senses. Their method was that of _introspection_, or the direct examination by the individual of his own ideas, and for the sake of noting their origin and composition from simple factors. The lineal descendants of these same English philosophers defined more carefully the process of _association_, whereby the complexity and sequence of ideas are brought about, and made certain conjectures as to its dependence upon properties and transactions in the physical brain. These are the three main philosophical sources of what has now grown to be the separate _natural science of psychology_. It will be noted that there are two characteristics which all of these studies have in common. They deal with the experience of the individual as composing his own private history, and tend to attribute the specific course which this private history takes to bodily conditions. It is only recently that these investigations have acquired sufficient unity and exclusiveness of aim to warrant their being regarded as a special science. But such is now so far the case that the psychologist of this type pursues his way quite independently of philosophy.