The main purpose of the paper is to prove that the essence of human nature lays primarily in person’s ability to reason; capacity that is uniquely human and allows people to make decisions that would shape their norms of conduct as shown in the works of ancient and modern philosophers. The complexity of finding the essence of the human nature comes from the fact that numerous academic disciplines have worked on the subject from their own point of view. There are so many existing theories that claim to have discovered the quintessence of human nature that a researcher may feel lost in what direction to look for answers. For the purpose of my paper I have narrowed down my arguments to the ideas supported primarily by the traditional Western philosophy in the works of Plato, Descartes and Kant.
According to the Traditional theory of human nature the humans are viewed as intellectual beings with a great capacity for reasoning. The concept is well presented in the dialogs of the ancient Greek philosopher Plato who believed that human soul consisted of three parts: “reason”, “spirit,” and “appetite” that were supposed to function in synchrony.  The rational portion of soul was located in the human brain and controlled the entire body. The spirit, responsible for the feelings and temper, occupied the chest of the human body. The third part of the soul called appetite was in charge of the basic human instincts such as thirst, hunger, or lust and was located in the stomach. Plato considered the reason to be the most important part because it was capable to control the primal urges and instincts and to guide the person in his actions.  He claimed that “the reasoning part should rule since it [was] really wise and exercises foresight on behalf of the whole soul.”  The best way to understand Plato’s concept is to imagine a patient at the hospital who is instructed to stay without food and water for twenty four hours prior to a major surgery. He is thirsty and hungry, but he consciously controls his temptations because he knows that such actions would jeopardize his health. As a result, he makes the rational choice to obey the doctors’ orders. By taking a closer look at Plato’s philosophy of the human nature it is fair to conclude that by reason, he meant the unique human capacity to think in complexity and to be in charge of his own actions.
Some people may argue with Plato that the reason does not always determine human actions. Often the basic instincts and emotions may take over and as a result a person can harm himself or hurt other people. We all have heard stories about the “crimes of passion,” when in a moment of rage and jealousy a person may kill his cheating partner. In his dialogs Plato addresses the possibility that the “spirited” and “appetitive” parts could dominate the human actions when the three portions of the human soul are not in “harmony.”  A properly functioning soul allows the reason to rule along with the spirit as an ally and the appetite kept under control.  In analyzing Plato’s view on reason it is obvious that human nature will thrive then the person is able to take control over the physical temptations, which leads to his rational decisions and choices.  I strongly agree that ability to reason is what makes a person unique, gives him the “right tools” to choose how to behave in certain situations and overall determines his norms of conduct in society.
Another big contributor to the philosophy of human nature and understanding the reason as the essence of human nature is the French thinker and mathematician Rene Descartes who became one of the leading thinkers of the seventeenth century. Initially, Descartes discarded all of his existing theories and beliefs in order to separate only the facts that he was certain of. In the process he discovered that he could doubt whether he had a body but he was absolutely certain that he had a mind. Descartes believed that the mind and body were two different things. The mind, according to Descartes, was a “thinking thing” and an immaterial substance, which was the essence of him that could doubt, believe, hope, and think.  So, Descartes argues, the mind, a thinking thing, can exist apart from the body, thus the mind is a substance distinct from the body, whose essence is the thought. To many people Descartes dualism may be obvious when we try to explain that physical characteristic of the human body, which the mind does not have. In addition, if person becomes physically ill, could his mind remind strong?
In his fundamental philosophical work Meditations on First Philosophy Descartes argued that the strict application of reason to all problems is the only way to achieve certainty in science and the explanation of human nature.  His most famous quote Cogito ergo sum, which translated from Latin means “I think, therefore I exist,” is not about Descartes infatuation with the process of thinking, rather it is about finding something that people would never doubt. Knowing that human senses could be deceiving, the only thing, Descartes says: “what we know with certainty is that we are thinking.”  Even if we are dreaming, or even trapped in a reality similar to the movie Matrix, we would never stop thinking and such knowledge confirms that we exist. Descartes successfully argues his point that the rational act of the human thinking is the actual proof of the human existence. In the arguments deriving from this statement, Descartes points out that prove the existence of his hair, face or even body, but he is certain that he has thoughts and ability to use reason. Descartes claims that these facts come to him as “clear and distinct perceptions.”  He argues that anything that can be observed through clear and distinct perceptions is does exist. He concludes that thought and reason must be the essence of humanity because they are clearly perceived.
In his works he argued that true knowledge about anything in this world, including the human nature, comes only through the application of pure reason.  He strongly believed that “while the external world operated by mechanistic principles, this [was] not the case with humans, who [were] guided by reason.”  The ability to reason is what separates the human being abruptly from the rest of nature, and suggests that what is specifically human cannot be investigated by the same principles that were utilized by the existing natural sciences.
Kant’s view on reason
In order to further understand human nature it is imperative to see the connection between reason and morality, which is eloquently argued in the works of Immanuel Kant.  According to Immanuel Kant (1724-1804) morality applies to all rational beings and a moral action is primarily defined and determined by reason and not by human senses.  In his major work Kant also believed
Another aspect of understanding the human nature is to explore the person’s ability to exercise his own free will. In philosophy, the doctrine that an individual, regardless of forces external to him, can and does choose at least some of his actions. In his dialogs, Plato introduces the idea the idea of man’s free will and power of choice as factor which determines a man’s parentage, his hereditary tendencies, his physical constitution and his early education, since all of these things are merely the effects of choices made in former lives. These choices also determine the man’s stage of evolution, show the position he should occupy in the well-ordered state, and indicate the particular virtue necessary for his immediate development. The whole problem of evolution, according to Plato, is one of ethics. As the ultimate aim of every man is to free himself from the tyranny of his lower nature, and as this can be accomplished only through the efforts of the individual, each man must start where he is, and develop that virtue which is most necessary for him.
What free will tries to account for is our introspective conviction that we are in control of many of our choices, and thus our destiny – that we are free to think and decide. We contrast this flexible, conscious control that we enjoy with the involuntary action of, say, our heartbeat or digestion, and with the instinctual imperative of a bird’s nest-building or a dog’s conditioned response. Our decisions are far more independent of nature than any animals; we are aware of our ability to think and of the consequences of our choices – we can claim responsibility for our actions. These are the meaningful differences that give rise to the concept of freewill.
Clearly not all philosophers agree on the definition of human nature, or on the laws that depend on it, but the pattern of reasoning is the same for all naturalistic systems of ethics. In his theories that all human beings have