St. Thomas Aquinas is one of the most recognizable names in Christian history and the Christian intellectual tradition. While generally held up as the perennial philosopher in the Catholic tradition, especially among Catholic realists, he is also loved—perhaps begrudgingly—by many in the Protestant world especially the so-called Reformed scholastics. There is also a lot of confusion about Thomas; was he an Aristotelian or a Neoplatonist using Aristotelian language? I definitely fall in the latter camp.
When it comes to understanding Aquinas’ cosmology and humanism, we have to understand some basic facts about the great church doctor. First is that the cosmos, which includes the earth, is hierarchal. Second is that there is distinctiveness and particularity to all forms of all, or as Aquinas says, “levels” or “genera” of life. Third, all the genera of life have particular souls to them.
Traditional Christianity, especially Catholicism, borders on “pantheism” to untrained eyes and ears. For every inch and crevice of the cosmos to have a soul sounds like pantheism to the theologically and philosophically uninitiated. But we must remember what I just outlined, all genera of life have particular souls to them; therefore, it is not a monistic pantheism because all do not share a common soul but distinctive souls through particular abilities as Aquinas explains in Quaestio Disputata de Spirituabilus Creaturis (or Disputed Questions on Spiritual Creatures).
In Summa contra Gentiles (4.11), Aquinas presents the clearest portrait of his ladder of existence, or the great chain of being. From here we will understand why Aquinas’ cosmos exudes with soul, that is life. For nothing can be being without soul; nothing can exist without a generative function. Thus, every level and genera of the cosmos is intimately interlinked with everything else; Aquinas has, as traditionalist Christianity demands, a deeply holistic, integrated, and interconnected world of intelligibility (as most of the Church Fathers defended and expounded upon in their theological works and homilies). Readers familiar with the classical tradition of philosophy will see obvious parallels with Plato, Aristotle, and Plotinus.
Aquinas begins by charting out the different layers of existence: There is non-living matter (earth), plant life, animal life, human life (all of which possess a soul), the angels, and then God as the Divine Mind and Generative Principle. Aquinas continues in extrapolating on the differences of these genera, “Different kinds of things produce in different ways, those on a higher level producing in a more interior way.” This is important for us to understand; higher forms of life, which technically begins with plant life, “produce in a more interior way.” That interior way of life is on account of the soul. As Aquinas explains, “In plants then there is this sign of life [anima]: that something within them moves towards some form…for the tree’s juice as it comes out of the tree first makes a flower, then later a fruit separate from the bark of the tree though joined to it, a fruit which, when mature, separates entirely from the tree and falls to the ground, there producing by virtue of its seed another plant.” Thus, the chain and cycle of life recapitulates.
Plant life, the vegetative soul, embodies the generative principle. But as Thomas also explains, without that non-living earth matter, a plant can never come into being. “The first beginning of production are outside: for the inner juices of the tree are sucked up by the roots from the earth which nourishes the plant.” The vegetative soul, then, is a particular soul because soul—anima—is life; anything capable of producing or generating life necessarily has the vegetative soul.
“There is another level of life above the plants,” Aquinas continues to explain, “that of animals endowed with sense-awareness so that they have a form of production peculiar to themselves.” Aquinas goes on to explain how all animals, possessing animal souls, have sensation. That means, minimally (unlike Bacon or Descartes or the Enlightenment materialists), animals possess the ability to feel and have spatial and emotional awareness. This was not an uncommon ancient, or Christian, view of animals—anyone with familiarity with St. Francis of Assisi or St. Bonaventure would be familiar with this deeply traditional Christian outlook.
Animals, however, possess in them the vegetative soul as well. For animals are also generative and reproductive creatures. Hence, the vegetative soul is part of their being. However, plants do not possess the animal ability of the soul.
“[T]he highest, most perfect level of life is that of the intellect, for the intellect can reflect upon itself and understand itself.” Unsurprisingly for those who are deeply familiar with Christian anthropology and philosophy—i.e., not Sam Harris, Richard Dawkins, Daniel Dennett and the so-called “New Atheists”—Aquinas marks out the highest life (human life) as self-reflective, conscious, and possessing the intellect. The intellectual soul, and the soul is the intellect in dogmatic Catholicism, is what distinguishes humans from the rest of creation. “The human mind, even though it can come to self-awareness, must still start by knowing outside things, and they can’t be understood without sense-images.” Here, Aquinas ties humans with animals—for humans are “rational animals.” However, the distinguishing feature of the distinction between pure animal and rational man is that man transcends, through his intellectual soul, mere sense-awareness and sense-imagery; man’s cognitive ability leads him to understanding and complexity like notions, concepts, and judgements.
If the vegetative soul is grounded in generation, and the animal soul grounded in sense-awareness, the rational soul is grounded in understanding. This is possessed only by humans and angels. However, while angels may be superior in their acme of perfection through superior intellects than humans, humans are superior to angels because they embody the tripartite soul: vegetative, animal, and rational. Humans have the ability to generate life (vegetative), have sense and spatial awareness (animal), and have the ability to understand (rational). This is why humans, and only humans, are “made in the image of God.” As Aquinas continues to explain, God is the all-encompassing soul which possess the genus of all generation (animal), all sensory awareness (animal), and knowledge of things (rational). Since humans are made in the image of the Trinity, the human soul is at once vegetative, animal, and rational.
St. Augustine, for instance, explained in The City of God how man’s rejection of his rational soul, his rational state, leads him to live like the beasts of the field. This is sinful, intrinsically so, because man’s nature is above and beyond the mere sensorial and animal. Dante, in Inferno, also gives nod to this reality of the tripartite soul in the seventh circle. By rejecting being made in the image of God, and separating himself off from the rest of creation, man’s estrangement from himself leads to consequences for the created world. Hence why the sinners take the form of vegetative plants. The degeneration of sinful man pushes his active life to that of being like the beasts of the field (living for mere stimulating pleasure against other objects) or in a vegetative state of existence without knowledge of good and evil and not even with spatial animal awareness. Man’s fall into sin and the state of sin—as Augustine and Aquinas both maintained—is by his rejection of his rational soul.
We can hopefully see the hierarchal chain of Aquinas’ Ladder of Being. At the lowest end of the ladder of existence is non-living matter, though this non-living matter does serve a function for life. Life, soul, cannot exist without it; hence why the cosmos (even in its “non-living matter”) is tied to soul because it is tied to plant life, or the vegetative soul. Above mere matter is plant life, which possesses the vegetative soul of generation. Above plant life is animal life with the animal soul of sense and spatial awareness and feeling. Animal life includes the vegetative soul as animals are capable of generation. Above animal life is human life, with its rational soul to know or understand but which also includes the animal and vegetative souls as humans have sense awareness and feeling (animal) and generative ability (plant). Above human life is the celestial realm.
Yet, since Aquinas was also a theologian and a Christian, the great Ladder of Being is also concentric (as it is in Plotinus). All existence emanated from the first generative act of creation which came from God. Thus, as all comes from God all directs back to God. This is why Aquinas concludes his discourse on the Ladder of Being by talking about God as the Divine Mind, the source of all existence, and the fountain of knowledge which calls creation to it.
In another section of Summa contra Gentiles (3.97-98), Aquinas states “you find the variety of things is achieved in steps: above inanimate bodies plants, above them unreasoning animals, and above them intelligent creatures.” He continues, “Variety of forms also brings with it diverse relationship to matter…And these diverse relationships to matter bring with them a diversity of agencies and capabilities of being acted on.” Aquinas’ diverse, interconnected, and intertwined cosmic ladder, cosmic whole, is not atomized and separated but deeply rooted together like a grand orchestra. Each individuated soul has its end, its purpose, to play—so to speak—in the grand symphony of life.
Thus, we see from Aquinas a living cosmos, a soul-filled cosmos, and an intelligible world of anima. It is a magical and mystifying cosmos, a cosmos of diversity and plurality, of particularity and instantiated uniqueness. All working together to reflect and embody beauty and goodness. That force which moves the Ladder of Being, and everything part of it, toward form is Love (since God is Love). Hence why the Ladder of Being is fulfilled in Love (who is God).
 Although non-living, it still serves a generative function. While non-living matter doesn’t reproduce after itself, it is essential for the start of the chain of life. In this sense, even non-living matter is attached to Soul.