This blog post explains Aquinas’s method of analyzing human nature through the relationship between philosophy and theology, exploring how humans, as social and political beings, form communities and states.

Philosophical Analysis of Human Nature

Attempts to analyze humanity have been made in various ways throughout history and across cultures. Plato attempted to analyze human nature while studying principles of the state and politics in his work ‘Political Philosophy’. During the Renaissance, Machiavelli also analyzed human nature while explaining ‘politics’, the most human of activities, in his ‘The Prince’. Among these numerous attempts, the most intriguing is Aquinas’s attempt to analyze humans from a religious perspective. Aquinas specifically analyzed humanity in his work, the Summa Theologica. Part I of the Summa Theologica deals with the attributes of nature and God, the Trinity of God’s personhood, and creation, while Part II describes and analyzes humanity as a moral agent. Aquinas attempts to analyze human nature through analogy in the relationship between God and humanity.

Aquinas’s Logical Analysis of Humanity

Before examining Aquinas’s view of human nature, it is necessary to first understand how he understood theology and philosophy. The medieval era in which Aquinas was active can be expressed by the phrase ‘philosophy is the handmaiden of theology’ (ancilla theologiae). Aquinas’s position on philosophy and theology can be found in a passage from his work, the Summa Theologica.
“Since grace does not destroy nature but rather perfects it, natural reason must assist faith. This is just as the natural inclination of the will must obey love.”
As this passage shows, philosophical truth and theological truth are in a mutually complementary relationship. Philosophical truth alone provides only an incomplete truth; it must be supplemented and perfected by theological truth, which is the ultimate truth. Conversely, theological truth is not separate from nature (philosophical truth). Since theological truth is revealed within nature, it cannot be perfected without nature (philosophical inquiry). The relationship between philosophy and theology can be academically analyzed in three ways. First, faith illuminated by natural reason (philosophy) and faith guided by the power of belief are mutually complementary and harmonious. Second, just as grace manifests in the natural world through the form of the image, theological truth also presupposes natural knowledge. Third, grace (theology) does not destroy nature (philosophy) but rather completes it.
Aquinas’s favorable attitude toward philosophy stems from his significant influence by Aristotle. Aristotle’s philosophy is grounded on the two principles of reason and experience. Successfully applying these seemingly incompatible elements of reason and experience to reality, Aristotle’s philosophy was well-suited to harmoniously uniting the somewhat disparate disciplines of theology and philosophy. Aquinas went a step further, opposing the limitations of naturalist philosophy—such as the world’s eternity and the separation of active intellect—from a religious standpoint, and developed them further. That is, he overcame what could be seen as the limitations of Aristotelian philosophy by borrowing the logic of religion.
In his work to grasp the essential structure of humanity, Aquinas drew upon Aristotle’s preceding research. He distinguished the soul, the substantial form of the human being, into intellect and appetites. The intellect was further subdivided into active intellect, passive intellect, speculative intellect, and practical intellect. The appetites were analyzed as natural appetites, intellectual appetites, and sensory appetites. This analytical framework is quite similar to Aristotle’s. Aquinas assigns grades based on mental level, explaining humanity’s unique position within the hierarchical order of all beings, including animals and plants. Furthermore, as beings created in the image of God, humans are guaranteed the most special status among all beings. Aquinas analyzes how such humans form communities and live within society.
Specifically, Aquinas’s position on human analysis is as follows. Beings are divided into material beings and immaterial beings. Material beings refer to animals, plants, and the like observable in the natural world, while immaterial beings are called ‘separate substances,’ such as angels and the Holy Spirit. Aquinas argues that humans possess the unique attribute of belonging to both material and immaterial beings. This is due to the nature of the human soul, which is the first principle of life (primum principium vitae) in living creatures and necessarily accompanies life. Simultaneously, the soul can exist independently of the body. However, this is not a dualistic argument presupposing the separation of soul and human existence. The soul’s capacity for independence means it can perceive all material things. This guarantees the soul’s unique status and connects to Aquinas’s intention to accurately grasp the principle of essence inherent in objects.
Aquinas distinguishes and indicates the grades of the soul, and this hierarchy represents the order of being. At the pinnacle of this hierarchy stands God, the pure actuality. God is a spiritual being in the most excellent sense, characterized by simplicity. God is an infinite and perfect being. Human beings, as creatures, cannot perceive God univocally or equivocally, but only analogically. All beings other than God are distinguished by essence and existence, essence and act, acts themselves are distinguished from one another, and the capacities for understanding and desiring are distinguished. Among beings other than God, the highest are the angels, who are not composed of matter and form. Angels are more perfect than humans and more like God. Beyond God and angels, other beings include human, animal, and plant souls. The vegetative soul is the primary principle of growth in life; the animal soul possesses not only the vegetative primary principle of life but also the sensitive primary principle. The human soul encompasses the vegetative principle of life, the sensitive principle, and the rational principle. This distinction reveals a hierarchy within the soul, divided into inferior and superior aspects.
The most unique feature of the human soul is its possession of the rational principle. Specifically, the rational principle is divided into intellect and desire. Knowledge through the intellect involves receiving the form of an object. Aquinas states that understanding the structure of human essence is equivalent to understanding the human soul. Since the human soul is the principle of reason, understanding intellect and desire equates to understanding humanity.
Intellect is divided into active intellect and passive intellect. The function of the active intellect is to grasp existence. That is, the human soul is like a blank slate before the act of understanding by the active intellect occurs. Alongside cognition, sensation must be considered. Humans perceive objects through the senses, and the intellect receives these sensed objects to form individual sensory impressions. A distinctive aspect of human cognition is that external objects are not merely reproduced through the cognitive faculties but are impressed upon them. For instance, even when perceiving inanimate objects like chairs or beds, these objects are not passive entities but exist as things capable of impressing themselves upon the human subject. Scholars studying the Summa Theologica describe the human cognitive process Aquinas explains as ‘knowing the nature of things’. The final stage of the cognitive process is judgment, through which the existence of the object is recognized. In other words, the cognitive process can be divided into ‘perception’ and ‘judgment’. Perception alone cannot determine right from wrong.
Aquinas’ epistemology can be broadly summarized as an exploration of the subject of cognition, the object of cognition, and the distinction between sensory and intellectual cognition that has persisted since antiquity. The medieval period in which Thomas Aquinas was active was dominated by a Christian worldview. To understand Aquinas’ epistemological thought, one must to some extent accept the religious mode of thinking.
First, Aquinas defines the subject of cognition as human beings. This is not an approach based on humanity as a vertical characteristic, but rather a new dimension that views humanity as the union of soul and body. Human beings are substances in which soul and body are united. Aquinas’s focus was on approaching the mechanism of cognition through these two dimensions: body and soul. Second, Aquinas distinguishes the object of cognition into two types: ‘sensory cognition’ and ‘rational cognition’. The object of sensory cognition is material things, while the object of intellectual cognition is the essence of things. Aquinas’s distinction between sensory and intellectual cognition does not assert the existence of different types of cognition at the same level, but rather a hierarchical division of cognition. That is, humans, as unified beings of soul and body, take sensory experience as the first basis of cognition. Aquinas divides sensation into two categories: external senses and internal senses. External senses include touch, sight, smell, taste, and hearing. The defining characteristic of external senses is their highly passive nature in the cognitive process. Their role is to receive the form of the external object as it is. Beyond external senses, humans possess an inherent function called internal sense. This is a capacity shared with the object, and through internal sense, the sensory image is formed.
Next is intellectual cognition. This refers to the process of recognizing the perception formed by the sensuous image as the essence of the object. Aquinas distinguishes intellectual cognition into ‘passive intellect’ and ‘active intellect’. The function of the active intellect is to transform the sensuous image into an object that can be thought of realistically through the act of abstraction. Furthermore, the active intellect abstracts forms from the sensible image and impresses them upon the passive intellect, enabling the human intellect to grasp the form of things. That is, through the active intellect, the sensible image becomes a concept, and this concept is then internalized within the soul and body through the passive intellect, forming knowledge.
Aquinas’s epistemology is significant in its detailed explanation of the processes of cognition and the formation of the intellect. Aquinas’s discovery is not revolutionary; rather, by distinguishing cognition into two layers—sensory perception and intellectual cognition—like Plato, it can be seen as partially adopting Plato’s position. Instead, through the concrete explanation of internal and external sensory organs, it provides a more detailed account of the process by which external objects are formed as sensory images and the process by which sensory images are shaped into forms and accepted as knowledge.
Next, we examine the characteristics of humans as ‘images of God’. According to Aquinas, human nature is formed according to the image of God. God is the fundamental cause of all beings. Humans are beings created according to the image of God. It is important to note that being created according to the image does not mean being ‘the same’ as God, but rather signifies a resemblance to some aspect of God. The divine image in humans has three stages. First, humans possess an inclination to understand and love God. This is common to all humans and can be seen as a reflection of the Christian worldview. For example, since all humans should possess a heart that loves God, it clearly distinguishes the ontological position of those without faith from those with faith. Second, humans imperfectly perceive and love God through their own strength alone. To perfect this, the spiritual power of grace and salvation is necessary. Third, humans possess the potential to fully know and love God, and loving God in this complete manner is called the ‘image of glory’. Beyond these three abstract concepts, humans also hold the highest rank among beings, excluding God and angels. This is because humans can perform the function of ‘cognition’ like God. The capacity for ‘cognition,’ absent in animals, plants, and objects, is, according to Aquinas, a unique ability possessed only by God and humans.
Before examining Aquinas’s social (political) theory, it is important to note that he distinguished human actions into ‘human acts’ and ‘human-like acts.’ ‘Human-like acts’ are activities performed by intellect and will. ‘Human acts’ refer to all activities other than ‘human acts’.

Aquinas’s Logical Analysis of Society (Politics)

Aquinas’s theory of society (politics) is quite similar to Aristotle’s theory, but he developed and inherited it in several areas. Aquinas accepts Aristotle’s proposition that ‘man is a social animal’ and develops his own social theory. Aquinas holds a teleological view of existence. That is, all creatures are born with an inherent purpose. Some beings inevitably fulfill the purpose they possess from birth. However, humans alone cannot achieve that purpose. Humans are inherently social and political beings; thus, they cannot fulfill their purpose of existence alone. Beyond the communal nature of humans, they possess desires. These desires become the driving force for pursuing their purpose. According to Aquinas, the universal human nature seeks the good of the individual while also desiring and yearning to achieve the common good. To achieve the common good, a group is necessary, and a group inevitably requires a leader to guide it. For beings pursuing the common good, both the group and the leader are essential.
In theology, the legitimacy of a leader is determined by the source from which governing authority is conferred. While governing authority is necessary for pursuing the common good, its legitimacy is conferred by God. That is, since the very nature of pursuing the common good is implanted in humans by God’s will, the legitimacy of the leader guiding it originates from God. Because the foundation of governing authority derives from God, the community known as the state is also created by God’s will and performs the function God has assigned.
To understand Aquinas’s view of the state, one must first understand Augustine’s. According to Augustine, because humans are born with original sin, a state without divine governance is a collective of corruption. Fallen humans have lost the true freedom granted by God; Augustine distinguishes between freedom and licentiousness. In a fallen state, humans are placed in a state of licentiousness, and to purify this, the state requires coercive means of sanction.
Aquinas critically incorporates the state theories of Aristotle and Augustine. He accepts Aristotle’s view of human sociability and also accepts Augustine’s justification for the state’s existence. For Aquinas, the state’s raison d’être signifies that the state is not merely an aggregate of licentious individuals, but rather a concept that exists a priori, created by human nature seeking to realize the true good. However, unlike Augustine, Aquinas argues that the state is not a consequence of human fallenness; even before original sin, humans formed societies and pursued the common good, and leaders or communities existed to guide them. According to Aquinas, the state is a complete society. Within the state, members are free to employ all means necessary to achieve the common good. The legitimacy of governing power is established in the direction of fulfilling the common good. Military force and the judicial system are means to eliminate all obstacles that could harm the common good. However, these coercive means must not infringe upon human mental activity. Since human spiritual activity relates directly to God, even if the state’s governance is legitimized by God, the spiritual purpose of humans can only be fulfilled by God and must therefore be left to the private sphere. It is the Church, not the state, that can guide spiritual activity. According to Aquinas, the Church becomes a society of a higher order than the state. However, the hierarchy between state and Church does not necessitate direct interference between them. The state and the church do not interfere with each other because their contributions to the community are different. However, he argues that while the supervision of earthly matters is entrusted to the king, the clergy perform the supervision of transcendent ends, and thus the king must submit to the clergy.