Nor is this hypothesis out of harmony with the notions otherwise entertained by the Stoics as to the nature of man. Plato and Aristotle had already fixed on the heart as the central organ of the lower powers; the brain they assigned to reason, with the view of distinguishing the rational from the mere animal soul. [497] When, therefore, the Stoics assimilated man’s rational activity to the activity of the senses, deducing both from one and the same source, it was natural that they would depart from Aristotle’s view. Accordingly, the various parts of the soul were supposed to discharge themselves from their centre in the heart into the several organs, in the form of atmospheric currents. Seven such parts are enumerated, besides the dominant part or reason, which was also called ἡγεμονικὸν, διανοητικὸν, λογιστικὸν, or λογισμός. These seven parts consist of the five senses, the power of reproduction, and the power of speech; [498] and, following out their view of the close relation of speech and thought, [499] great importance is attached to the power of speech. [500] At the same time, the Stoics upheld the oneness of the substance of the soul with greater vigour than either Plato or Aristotle had done. Reason, or τὸ ἡγεμονικόν, is with them the primary power, of which all other powers are only parts, or derivative powers. [501] Even feeling and desire they derive from it, in direct contradiction to the teaching of Plato and Aristotle; [502] and this power is declared to be the seat of personal identity, a point on which former philosophers had refrained from expressing any opinion. [503] [B. The individual soul and the soul of the universe.]