Therefore, a plant has a soul. It contains a factor that manages the plant and keeps it functioning as a whole. It oversees the plant’s growth and reproduction cycle. Aristotle likened the simple organs of a plant to those of more complicated animals. The roots of a plant, he claimed, are like the mouth of a higher functioning animal. Both are used to obtain food.
The Islamic Animal Soul
In his treatise An-Nafs (The Soul), the Islamic Philosopher Ibn Sina (980-1037) explains that because we observe a living body acting a certain way, it is its action that prods us to infer that each living body has a soul. For example, when we see an animal reproducing, eating, moving and perceiving, we can infer that these actions come from some principle within the animal. A cow grazes, pulling up grass for nourishment. Unlike a rock, or a dead cow for th
at matter, this ingestion of food, according to Sina, is indicative of an animal soul.
Similarly, when we see a pine tree getting taller and producing cones for regeneration, we can infer from these actions that the tree has a plant soul. Its soul is more limited than an animal soul. Animals have locomotion and perception of the outside world.
Another Islamic thinker, Ibn Rushd (circa 1180), claims in Tahafut Al-tahafut (The Incoherence of the Incoherence, trans. S. Van Den Bergh, 1954) that the animal soul even has a form of imagination. He says that when a sheep has a notion that a wolf is to be avoided, the animal is using this power of its soul.
The Medieval Animal Soul
Following in the tradition of Thomas Acquinas (1225-1274), the Scholastics of the Middle Ages held almost the same views about the souls of plants and animals a
s did Aristotle. They believed that the origin of animal life is the soul, which is superior to the plant soul only capable of nutrition, growth, and reproduction.
Acquinas posited that the animal soul is sensitive. It is capable of perception of reality, motion, and an additional quality named appetition (Summa Theologica: Rational Psychology). Because of this quality in its soul, an animal is able to strive toward things it finds desirable and avoid what is not. This quality is reminiscent of Ibn Rushd’s notion mentioned above. A bird, for example, will not hunt just any food. It will seek the kind of food that it finds most desirable and suitable for its beak (a finch's bill versus that of a duck or sandpiper). In many ways, this sounds like what modern biologists refer to as instinct.
Descartes and the Animal Machine
Because of his mind-body dualistic philosophy, Descartes attributes no real mental functions to animals. To him, it is scientifically ridiculous to claim they have souls. Animals are nothing more than complicated machines which operate according to instinct. He quarreled with the ancients from a Catholic Christian point of view.
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