The Stoic teaching becomes peculiarly interesting, when it treats of Man; and the line it here follows is decided by the tone of the whole system. On the one hand, the Stoic materialism shows itself most unmistakeably in the department of anthropology; on the other hand, the conviction that all actions must be referred to active powers, and all the several active powers to one original power, can not be held without leading to a belief in the oneness and in the regulating capacity of the soul. Not only does it follow, as a corollary from the materialistic view of the world, that the soul must be in its nature corporeal, but the Stoics took pains to uphold this view by special arguments. Whatever, they said, influences the body, and is by it influenced in turn, whatever is united with the body and again separated from it, must be corporeal. How, then, can the soul be other than corporeal? [486] Whatever has extension in three dimensions is corporeal; this is the case with the soul, since it extends in three directions over the whole body. [487] Thought, moreover, and motion are due to animal life. [488] Animal life is nurtured and kept in health by the breath of life. [489] Experience proves that mental qualities are propagated by natural generation; they must, therefore, be connected with a corporeal substratum. [490] As therefore, the mind is nothing but fiery breath, so the human soul is described by the Stoics sometimes as fire, sometimes as breath, at other times, more accurately, as warm breath, diffused throughout the body, and forming a bond of union for the body, [491] in the very same way that the soul of the world is diffused throughout the world, and forms a bond of union for the world. [492] This warm breath was believed to be connected with the blood; and hence the soul was said to be fed by vapours from the blood, just as the stars are fed by vapours from the earth. [493] The same hypothesis was also used to explain the origin of the soul. One part of the soul was believed to be transmitted to the young in the seed. [494] From the part so transmitted there arises, by development within the womb, first the soul of a plant; and this becomes the soul of a living creature after birth by the action of the outer air. [495] This view led to the further hypothesis that the seat of the soul must be in the breast, not in the brain; since not only breath and warm blood, but also the voice, the immediate expression of thought, comes from the breast. [496] [(2) Divisions of the soul.]