The Aristotelian/Thomistic account of the soul is part and parcel of Natural Philosophy. It makes use, therefore, of the notions of matter and form, potency and act. Aristotle defines the soul as the act of a natural body with the capacity for life; and as the first act of a natural organic body. Soul is thus the formal cause of the animal, the efficient cause of its motions, as well as its final cause. The body cannot be the principle that accounts for life, since a body, when deprived of life, is still a body, but not alive. The body is matter to the soul, and soul is form or act to the potentiality of the body. Moreover, the matter, i.e. the constituents that make up the body, are constantly changing while the animal persists. The animal's form or functional organization, i.e. organization of material parts by which an animal accomplishes its vital functions, remains the same. This form is the animal's soul.
There is a hierarchy of vital functions, and thus of different kinds of souls. First of all, there is the vegetative soul which accounts for the functions of nutrition and reproduction. Plants have only this kind of soul. Next, there is the sensitive soul, by which higher animals perceive and respond to their environment. This kind of soul, for some animals, also includes the power of local motion. Finally, there is the rational soul, by which humans are able to use speech and have abstract thoughts. In all of the higher kinds of organisms, the functions that were performed by lower kinds of souls are performed by the higher. Thus, there is only one soul in any particular animal even though it is has the same vegetative capacities as plants. The vegetative functions, which are performed by a plant's soul without sensitive functions, are also performed by the sensitive soul. Likewise, the rational soul is the principle also of sensitive and vegetative functions of human beings. Thus there is a hierarchy of souls and of vital functions, such that the higher souls subsume the lower, but the lower vital functions are necessary for there to be higher ones. The higher are never found without the lower, but the lower are found without the higher. Moreover, there is an interaction between the capacities that characterize higher and lower souls: a lion uses sight to find food, and moves toward the lamb it spies, which it then eats and digests so that it may chase other prey.
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