General Philosophy of Classic Philosophers by Jonathan Dolhenty, Ph.D.
Besides God, the spiritual substances are the angels and human souls. Angels are not destined to inform any matter; the human soul, on the contrary, is ordered to be the form of the body. Hence the question arises as to the nature of the soul and its relations with the body.
In regard to the first question, at the time of Aquinas, the Averroists held that “the agent intellect” was a form existent per se and that it was separated from human souls, in which, however, it made its appearance occasionally in order to impress the intelligibles on the passive intellect. The logical conclusion in this theory is that the human soul will perish when the conditions of the body make impossible the presence of the Unique Intellect.
Aquinas was always a strong opponent of Averroism. He rejected the unity and transcendence of the agent intellect not only for theological but for philosophical reasons.
As Aquinas observes (1), he who receives an intelligible form does not thereby become an “intelligent being.” For instance, a house which receives the intelligible form of the idea of the artist, is intelligible but not intelligent.
Man not only is intelligible but also intelligent; he is intelligent, because he make intelligent operations. The principle of these intelligent operations, therefore, must be the soul itself and not a separate intellect. (2)
The second question deals with the relationship of the human soul to the body. In man there are many operations — vegetative, sensitive, and intellective. Now, unquestionably the intellective operations are performed by the rational soul. But who performs the others?
Platonic-Augustinian philosophy solved the question by admitting a multiplicity of inferior forms which are subordinated to the rational soul. Thus there was a sensitive form as well as a vegetative form.
Aquinas, following Aristotle in this matter, denies any multiplicity of substantial forms in the same individual. The form for man is one as is the form for any individual thing; in man this form is the rational soul. It is the principle of all operations, whether material or spiritual.
We know that the one soul understands and performs all the operations. We express this identity of the subject when we say: “I understand and I feel, and I see.”
Proper to the human soul is the understanding, which does not need the cooperation of any organ in its operations. But the human soul is also the “form of the body”; and just as every form is the principle of all the operations of the informed matter, so also the human soul is the principle of all operations performed by the body through its various organs. (3)
The doctrine that “the soul is the form of the body” gives rise to another difficulty, which seems to spring from the same principle of matter and form taken from Aristotelian metaphysics.
According to Aristotle, the forms of natural bodies depend on the conditions of matter, so that when these conditions become unfit the permanence of the form is no longer possible; then it will be corrupted and another form will take its place. Hence the doctrine of the soul as the unique form of the body seems to lead logically to the mortality of the human soul.
Aquinas overcomes the difficulty with the same Aristotelian principles. The operations of any being follow from its nature; thus any form leading only to organic operations is bound to matter and follows the conditions of matter, as, for instance, the animal soul, which is corrupted with the organism. But the human soul has superorganic operations.
The intellect does not need any organ in its understanding; hence the human soul is a superorganic substance, not dependent for its being upon any matter. And despite the fact that the human soul is the form of the body, it will last as a separate substance of intellectual nature, even when the conditions of the body render impossible the functioning of the soul as the form of the body. (4)
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Thus the doctrine of Aquinas concerning the soul in general and the human soul in particular, may be summed us as follows:
When the form in matter is the origin of immanent actions, it gives origin to life and as such is more particularly called the “soul.” There is a vegetative soul, such as the principle of plants, whose activity is fulfilled in nutrition, growth, and reproduction. Superior to the vegetative is the sensitive soul, which is present in animals; besides the processes of nutrition, growth and reproduction the sensitive soul is capable of sensitive knowledge and appetition. Superior still to the sensitive soul is the rational soul.
The rational soul is created directly by God; it is distinct for each man; it is the true form of the body. The human soul performs the functions of the vegetative and sensitive life, but besides these functions it has activities which do not depend upon the body, i.e., understanding and volition.
The intellect and the will are the faculties of the soul, the means through which it operates. The intellect has for its object the knowledge of the universal, and operates by judging and reasoning. The will is free; that is, it is not determined by any particular good, but it determines itself.
From an analysis of the intellect and the will, Aquinas proves the spirituality, the simplicity, and the immortality of the soul. The intellect has, in fact, for its proportionate object the universal, the understanding of which is a simple and spiritual act. Hence the soul from which the act of understanding proceeds is itself simple and spiritual. Since it is simple and spiritual, it is by nature also immortal.
The same conclusion is reached through an analysis of the will, which, as we have said, is free, i.e., not determined by any cause outside itself. In the physical world everything is determined by causal necessity, and hence there is no liberty. The faculty which is not determined by causal motives declares its independence of these causes and hence is an immaterial faculty. The soul upon which such a faculty depends must be of the same nature as the faculty; that is, the soul must be immaterial.
The human soul, since it is immaterial and performs acts which are not absolutely dependent upon the bodily organs, does not perish with the body — although, as Aquinas says, the soul separated from the body is not entirely complete but has an inclination to the body as the necessary instrument for its complete and full activity.
(1) Contra Gent., II, 76).
(2) Summa Theol., Part I, q. 79, a. 4 and 5).
(3) Summa Theol., Part I, q. 76, a. 1; Contra Gent., II, 57 and 58).
(4) Summa Theol., Part I, q. 75, a. 6′ Contra Gent., II, 78, 79 and 82).