St. Thomas Aquinas and the Nature of the Soul

It is said that St. Thomas Aquinas referenced Avicenna about 400 times in his works. I would say that counts as being very influential on someone, and we will see just how much influence Avicenna had on the Thomistic interpretation of the soul.

St. Thomas Aquinas’s anthropology of the soul accounts for three divisions of the soul: the Vegetative, the Sensitive, and the Intellectual/Rational. These divisions are set in a hierarchy: plants have only a Vegetative soul–the power that allows for basics of existence–nourishment, growth, reproduction; the Sensitive is inherent in all animals, irrational and rational (human) and encompasses sensation and perception and movement; and the Intellectual/Rational soul is specific to human beings only and is the soul responsible for reason and thinking. The hierarchy is such that human beings possess all three kinds of soul, irrational animals (all other animals besides humans) possess both the Sensitive and Vegetative powers, and plants only have the Vegetative. (We can see that these divisions and hierarchy are indeed imitative of that of the Avicennian psychology of the soul.)

As we said, all living beings have at least the Vegetative soul:

“The lowest of the operations of the soul is that which is performed by a corporeal organ, and by virtue of a corporeal quality. Yet this transcends the operation of the corporeal nature; because the movements of bodies are caused by an extrinsic principle, while these operations are from an intrinsic principle; for this is common to all the operations of the soul, since very animate thing, in some way, moves itself. Such is the operation of the ‘vegetative soul.'” (Summa Theologica, Q. 78, Art. 1)

St. Thomas Aquinas equates the lowest form of soul with the corporeal nature of a living thing. That is not to say, as we can see from the text above, that this Vegetative soul is reliant on the body, but rather that it “acts only on the body to which the soul is united.” (Q. 78, Art. 1) The Vegetative power of soul is that part which accounts for the basics of life for any animate (ensouled) being. St. Thomas acsribes three powers to the Vegetative soul–Nutritive, Augmentative, and Generative. These powers are, as should be apparent, those which work to keep a living being alive, growing, and continued in existence. The Nutritive power accounts for food and nourishment, without which a living thing would cease to be. It seems very mundane, but if an animate creature is not nourished/fed, it decays/dies. The Augmentative power is associated with growth, which is necessary for all living beings in order to reach the third power of the Vegetative soul, the Generative. The Augmentative (or growth) prepares and readies the animate being for continued existence through reproduction, or self-preservation. The corporeal natures of all living things rely on the basic processes/powers of the Vegetative soul. We can see that without the Vegetative soul, existence of animate beings would be, well, non-existent.

The second division of soul is what St. Thomas Aquinas calls the Sensitive soul:

“There is another operation of the soul, which is indeed performed through a corporeal organ, but not through a corporeal quality, and this is the operation of the ‘sensitive soul.'” (Q. 78, Art. 1)

For St. Thomas, the Sensitive soul does not work through corporeal qualities (like that of the Vegetative), but does work through what he calls “corporeal organs.” These organs are the senses. The Sensitive soul encompasses the powers of perception and (willful) movement. Perception here relies not only on the external senses–seeing, hearing, smelling, touching, and tasting–but also on internal senses as well–imagination, common sense, estimation, and the memorative. (As we can see, this delineation is strikingly similar to the Avicennian notion of both external and internal senses associated with the Animal Powers.) The use and powers associated with the senses, both internal and external, allow animals (irrational and human) to go beyond themselves and interact with the world around them. Unlike plants (that have only a Vegetative soul), animals use the powers of the Sensitive soul to take care of the basic needs of existence (the vegetative). Animate beings endowed with Sensitive souls are able to take in knowledge and conduct willful movement by use of the five external senses. The internal senses represent the ability of those with Sensitive souls to store, use, and process that obtained knowledge and information. For St. Thomas, the Sensitive soul “regards a more universal object–namely, every sensible body, not only the body to which the soul is united.” (Q. 78, Art. 1)

And thus we come to the highest form of soul–the Rational (Intellectual):

“There exists, therefore, an operation of the soul which so far exceeds the corporeal nature that it is not even performed by any corporeal organ; and such is the operation of the ‘rational soul.'” (Q. 78, Art. 1)

The rational soul is the power that is unique to the rational animals; the rational soul is only found in human beings. It is this soul that accounts for the ability human beings have to reason and engage in higher order cognitive function (knowledge of universals). It is indeed what makes us human after all. Human reason, intellect, is inherent in the rational soul. It is a power that goes beyond the mere collection and retention of knowledge (through sensory perception)–as with the Sensitive soul; it accounts for our non-sensory knowledge, our knowledge of universals, and our ability to be self-aware. As St. Thomas tells us, the Rational soul “regards a still more universal object–namely, not only the sensible body, but all being in universal.” (Q. 78, Art. 1) Unlike both the Vegetative and Sensitive souls, which both need the body in their own ways, the Rational soul is not reliant on corporeal natures. It is known to itself by virtue of itself; the Rational soul transcends the corporeal qualities (i.e. bodily functions and the senses), maintaining an existence that is not dependent on the body. (Again, if we reference the posts on Avicenna, we will see much of the same line of thought regarding the human soul.)

If we take what we have here on the Thomistic anthropology of the soul and review the posts on the Avicennian psychology, I think it would be very apparent that we can make the connections between the two philosophies. It is indeed the case that St. Thomas Aquinas was greatly influenced by the philosophy of Avicenna.