63. Applications of Physics to Psychology. -- Nature and Origin of Knowledge. -- Before the soul acquires actual knowledge it resembles a sheet of paper on which no letters have yet been traced. Sensation is the source of all knowledge. Sensation, by practice, gives rise to memory; from repeated acts of memory comes experience; from reasonings on experience arise the concepts by which we pass beyond the bounds of experience; the combination of these concepts is science. It seems, therefore, that thought is only an elaborated or collective sensation. The object of our universal ideas (lekton) comes between the real thing (tugchanon) and the word (phônai, sêmeion).

The Stoics believed in the possibility of certitude and defended it against the Sceptics. Neither party, however, place the question on its proper basis: the analysis of our cognoscitive activities. They reduce it to a corollary in ethics. Whilst the Sceptics deny the existence of certain knowledge, because they consider it superfluous in their system of ethics, the Stoics affirm it as indispensable for morality. Without certain knowledge, they say, it would be impossible for us to conform our conduct to true ideals. Practical necessity is thus made the decisive argument against all scepticism.

The criterion of certitude is purely subjective. It is the convincing force (katalêptikon) inherent in a representation, the power which knowledge possesses of winning our firm adherence. By a strange contradiction, they attribute this power to concepts (lekta) and not to sensations, though the general concept must be essentially false since it corresponds to nothing corporeal (62, 1).

The Will. -- Determinism in human acts is only a particular application of cosmic determinism. The Stoics try in vain to safeguard liberty by confounding free with voluntary acts.

Nature of the Soul. -- The soul is an emanation from God or the world-soul: it is simply a little warm air, a pneuma. Grecian authorities on physiology and medicine had long sought an explanation of normal and abnormal vegetative functions in warm air (pneuma).^{1} The Stoics improved on this conception by confounding the pneuma with the soul itself, -- activity with the principle of activity. We see traces of this confusion in their theories relative to the soul's origin location, constituent parts, and future life. The soul of the infant is a particle of matter separated from the soul of the parents. The soul has its locus in the breast where respiration produces warm air. From the breast also comes forth speech which is the immediate expression of the thinking soul. By means of the air which the heart sends through the organism the soul occupies and penetrates the whole body. (See above, the krasis di' honôn.) The Stoics seem to have multiplied the parts of the soul just as the whim seized them. Reason, however, was regarded by all as the directing part (hegemonikon), the principle of the Ego and of personality. At the end of time when the world will be consumed by fire, all human souls will be absorbed in the Divine pneuma. It follows then that the soul will for some time survive the body. Is this survival the privilege of the virtuous, or is it the common destiny of all? Their answers to this question are contradictory. The whole theory of survival is a concession to moral exigencies at the expense of their materialist principles.

{1} SIEBECK, Geschichte der Psychologie, i., 2 (1884), p. 133 cf. 53 (1), Aristotle's idea of the pneuma.