Aristotle on the Differences between Plants, Animals, and Human Beings and on the Elements as Instruments of the Soul (De Anima 2.4.415b18)

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WHY DO ALL ANIMALS POSSESS SENSE PERCEPTION while plants don't? Should the difference in quality of life between human beings and wolves be explained by supposing that wolves have degenerated souls? This paper argues that for Aristotle differences in quality of life among living beings are based on differences in the quality of their soul-principle together with the body that receives the soul.

A new interpretation is proposed of On the Soul 2.4.415b18: "For all the natural bodies are instruments of the soul," against all current interpretations. Aristotle there means that each of the four sublunary elements can be a part of the instrumental body of a soul. The paper continues with discussing the way in which Aristotle connects the several sublunar elements with different levels of life activity, and the troublesome passage in Generation of Animals 3.11.761b22, where Aristotle speaks about a fourth category of living creatures related to the fourth sublunary element, Fire, and the region of the Moon.


The Soul Principle as the Basis for Difference in Quality of Life. How does Aristotle explain differences in level and quality of life? The answer seems obvious. Aristotle divides the realm of (sublunary) living creatures into three subrealms, plants, animals, and human beings. To each subrealm he assigns a different soul-principle. Plants have a vegetative or nutritive soul; animals have a sensitive soul; human beings have a rational soul. (1) For Aristotle there is a difference in "value and lack of value" between these levels of life. For plants produce nothing but seed, for the continuation of their own kind. Animals, however, all have a certain level of "knowledge," though for some kinds this is restricted to the lowest level of perception. This worthiness of only a form of perception may seem minimal in comparison with the human mind, but compared with the condition of a plant or a stone it is something astonishing. (2)

On this view, the soul-principle is the basis for difference in level of life. But this signally fails to do justice to the great variation within each of the subrealms. It also falls to explain why a sensitive soul never manifests itself in a plant or tree. Another question to be decided is whether perhaps a monkey or a wolf or fish possesses a human soul which has degenerated. (3)


"The Body that Receives the Soul" as the Basis for Difference in Quality of Life. There is also another side to the problem. The soul as "the first entelechy of a natural body that potentially possesses life" (4) is never "without soma," says Aristotle. (5) So it is relevant to pay attention to "the body that receives the soul," for "a craft must use its instruments, and a soul its body." (6) A carpenter does not hammer in a nail with a flute. A flautist has no use for a hammer. It thus seems as if the body with which the soul is connected imposes restrictions on the soul and that the quality of the body that receives the soul determines the quality of life (just as the menstrual fluid can only be fertilized by semen of a male partner of the same species).

Note, too, that it is not the structure of a visible animal body which can impose restrictions on a soul-principle. (7) For this visible body is itself the product of an animal soul-principle. And the same applies to the structure of a visible plant body and a visible human body.

Aristotle was aware that it is necessary to speak about the specific quality of "the body that receives the soul," and that it is unsound, as the Pythagoreans did, to put all souls in the same category (8) and not explain why the various kinds of animals and humans (and plants, Aristotle would add) are so different.


The Soul in Combination with "the Body that Receives the Soul" is the Basis for Difference in Quality of Life. However, a famous passage in his great work Generation of Animals (2. …