Following up on Aquinas’ Ladder of Being, we move into a related concern that the good doctor dealt with in Quaestio Disputata de Anima (Disputed Questions of Life or the Soul). In this particular disputation, Aquinas is dealing with what distinguishes souls. The question at hand, which follows from an earlier disputation in another text concerning the soul as potential or ability, is whether a soul is distinguished by objects or in its abilities (or powers).
Aquinas argues that what distinguishes souls is not objects but its abilities. It is again important to understand the interlinked cosmology and world of Aquinas as opposed to our post-Baconian functional dualism. Objects of the soul, which seem to mark out its distinctiveness, like color differences, “correspond sometimes to different abilities—hearing and sight—and sometimes to only one—imagination of mind. So differences in object don’t cause diversity in ability” he writes. Thus, abilities are what distinguish souls. Moreover, these differentiated abilities are in origins rather in object or matter as he explains.
In trying to tease out his position, Aquinas argues “every genus has its fundamental pair of opposing properties. So if we are going to differentiate sense powers by diverse genera of passive qualities, there should be different sense-powers wherever there are diverse pairs of opposing qualities.” Passive and active senses are not the result of objects, but rather the differentiated function of abilities. The encounter with world objects don’t distinguish the soul either, rather, the encounter with objects in the world magnifies the differentiated abilities inherent in the soul’s nature. For example, it is not the tree that causes the distinctiveness of souls. The tree may magnify the ability of sight which is an ability of the level of life associated with “sense-awareness.” But the ability of the soul resides in itself and not with external objects. The soul is within.
Aquinas establishes, predictably, “three levels to soul activity.” Those three levels of soul activity are (1) vegetative powers, (2) sense powers, and (3) intellectual powers, each corresponding with the distinctiveness of the soul based on its ability. What marks out the distinctiveness, or what distinguishes, souls are in their power or ability. The powers of the vegetative soul mark it out as different from the powers of the animal (or sense) soul and the powers of the intellect mark it out as different from the powers of the animal and vegetative soul.
Concerning the three levels of soul activity, Aquinas argues that soul activity can be broken down in two ways: “in a manner of acting and in what gets done. In manner of acting all soul-activity transcends the action of workings of nature in non-living things, because soul-activity is life-activity and living means self-moving, so that all soul-activity comes from interior agency.” Note, here, there is sharp distinction between Catholic and Reform anthropology. Regarded the Fall of Man, Catholicism—including Augustine—never taught “total depravity.” Why? Because “all soul-activity comes from interior agency.” Total depravity would mean a complete absence of interior agency which had been lost as a consequence of the Fall in Calvin’s outlook (cf. Institutes of the Christian Religion, 4.14.4). Calvin and the Calvinists argue that human agency is totally and completely moved by the Spirit of God only; hence the inevitable conclusion of the hard Calvinists that God is the author of evil because a depraved human has no interior agency to self-move on its own. Hence, those souls who move toward evil things do so because God wills them to do so. Hence why those souls who move toward goods things do so because God wills them to do so. But this is not the time and place to enter into this anthropological discussion. It should be flagged for educated readers, however.
Since soul-activity for Aquinas is in movement, power, or ability, the powers of the vegetative soul are what Aquinas considers “natural powers.” The vegetative soul is what all life possesses, including fallen humans, and can never be taken away. Thus, in Catholicism, the fallen human is corrupted down to the level of the vegetative (or animal) soul before being restored to its rational state through baptismal regeneration. “[T]he soul’s potentials for such activities” (the activities hitherto described) “we call its vegetative powers. These comprise generative powers which bring individuals into existence, powers of growth which cause them to attain their proper size, and nutritive powers which conserve them in existence.”
The vegetative powers of the soul are the lower powers of power, corresponding with generation, growth, apogee, etc.
“[A]t the level of sense-awareness,” Aquinas writes, “which suffices for animal life, five things are needed.” Those five things, those five powers or abilities, are: (1) particular senses (the five senses: touch, taste, smell, hearing, and sight), (2) general root sensitivity, (3) fancy, (4) judgement, and (5) memory. Particular senses and general root sensitivity are universal to all animals. Fancy, judgement, and memory, while also universal to the animal soul, is where the cleavage between animals and humans will diverge. For animals, fancy, judgement, and memory come from their natural instincts endowed by God through their animal soul. However, for humans, these sense powers are the result of “research and discussion,” and by “enquiry.” Memory, for instance, “in other animals work without enquiry, but in human beings with enquiry and study: so that human beings not only have memory but reminiscence.”
The highest form of sense-awareness is in motive power and affective and aggressive emotions. For Aquinas, animals, even dogs and cats, are motivated by affectivity. Their aggressive abilities is not a deficiency of love, but points—perhaps in corrupt form—to affectivity. Affective emotion “empower[s] us to enjoy things delightful to the senses.” By contrast, and again, refer back to Aquinas discussed “opposing properties,” aggressive emotion “fights to repel obstacles and gain access to what [it] naturally enjoys.” Aggressive power, actus, or action, aims at enjoyment. Hence, affective emotion stands superior to aggressive emotion and the motive power of sense awareness is enjoyment. (Aquinas builds from Augustine’s arguments in The City of God about how peace and felicity is what motivates all action, even war or other aggressive actions.)
We see, then, that the distinguishing characteristic of the plant and animal soul is in their powers. Not their object. Where the plant soul has the power of generation and growth, the animal soul has the power of sense-awareness and the five powers of sense-awareness Aquinas outlines and elaborates on. Most essential to the animal soul is its power of particular senses, moving all the way up to natural instinct (beast) and cognitive ability to form notions and concepts (rational animals). This split, this cleavage, between natural instinct and the power of enquiry is what distinguishes the rational soul from the animal and plant soul—which Aquinas calls “intellectual powers.” For animals as animals, the telos of their soul is in affectivity (without understanding).
The rational soul is marked by intellectual powers, and these powers are in “agent intellect” and “receptive intellect.” Agent intellect is that soul-activity of movement, of voluntas, of will or agency. Agent intellect is the rational soul desiring something. Receptive intellect is the part of the rational soul that takes in what agent intellect is seeking after. Again, we see the opposing qualities of the intellect at work. Agent intellect desires, receptive intellect receives what is desired. For Aquinas, what the rational soul desires (agent intellect) is knowledge (consummated in the receptive intellect). When agent intellect finds what it desires, and receptive intellect takes in what is found, the intellect is satiated. This, for Aquinas, is Truth. Agent intellect seeks the good and desires to understand the good. “Each soul,” he writes, “has a primary goal—for the human soul what it understands to be good—which all its other goals serve—so that what is sensed as good serves what we understand to be good.” The rational soul’s power, that which most definitively distinguishes it from plant and animal souls, is the “ability to understand.” The power of the rational soul is understanding; man is an understanding or thinking being.
Thus, we see from Aquinas the three levels of life distinguished by the powers of the soul. The plant soul aims to generate life. The animal soul aims to embody, or feel, affectivity. The human soul aims to know the good (i.e. God). The human soul also includes the ability to generate life and to embody affectivity all through knowledge of the good.
To conclude, what are the three levels of life then? They are the three powers of the soul. Plant life; animal life; and rational life. Each level of life has powers particular to its ends which intersect together, in their own ways, to reflect and embody the beauty, love, and goodness of God (see Summa contra Gentiles, 3.97-98). What permeates the cosmos? What is the mediation that link the powers of the soul to their particular ends? Love. Love generates. Love enjoys. Love understands. Love calls all things to it through their distinctive abilities. Love, then, is the World-Soul for Aquinas which moves all things in the symphonic waltz of life.
 Anima, in Latin, means life. It is also the word often translated as soul since soul is that which gives life. Hence I have translated the Latin title as including both words.