To that something corresponds
in the case of animals the soul and in this it follows the order of
nature; all natural bodies are organs of the soul. This is true of
those that enter into the constitution of plants as well as of those
which enter into that of animals. This shows that that the sake of
which they are is soul. We must here recall the two senses of 'that
for the sake of which', viz. (a) the end to achieve which, and (b) the
being in whose interest, anything is or is done.
We must maintain, further, that the soul is also the cause of the
living body as the original source of local movement. The power of
locomotion is not found, however, in all living things. But change
of quality and change of quantity are also due to the soul.
Sensation is held to be a qualitative alteration, and nothing except
what has soul in it is capable of sensation. The same holds of the
quantitative changes which constitute growth and decay; nothing
grows or decays naturally except what feeds itself, and nothing
feeds itself except what has a share of soul in it.
Empedocles is wrong in adding that growth in plants is to be
explained, the downward rooting by the natural tendency of earth to
travel downwards, and the upward branching by the similar natural
tendency of fire to travel upwards. For he misinterprets up and
down; up and down are not for all things what they are for the whole
Cosmos: if we are to distinguish and identify organs according to
their functions, the roots of plants are analogous to the head in
animals.
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