"What, then, is the animal? First of all, a system of plant-souls. The unity of those plant-souls, which unity nature itself produces, is the soul of the animal. Its world is therefore partly that of the plants — its nourishment, for instance, it receives partly through synthesis from vegetable, and through analysis from animal nature — and partly that of the animals, whereof we shall speak directly. ... Each product of nature is an organically in-itself completed totality in space, like the plant. Hence, the unknown x which we are looking for must also be such a whole or totality, and in so far it must also have a principle of organization, a sphere and central point of this organization; in short, the same which we have called the soul of the plant, which thus remains common to both. ... The animal is a system of plant-souls, and the plant is a separated, isolated part of an animal. Both reciprocally affect each other."