Aristotle was really, for a long time, the apex of philosophical thought. St. Augustine and most certainly St. Thomas Aquinas developed and built on his ideas, adding Christian thoughts. St. Justin Martyr did, too, and he even claimed that Aristotle was a Christian, he just didn’t know it – rather, he was influenced by the divine Logos.
Aristotle is probably a good place to look.
[Aristotle’s] treatise is entitled On Soul, and is part of his study of the animal kingdom, because that is what humans in their full nature are—animals. And Aristotle focuses on the soul because he starts with the traditional Greek idea that there is some internal cause which separates animate from inanimate things in the world of nature. The name for this cause is “soul” (psyche).
That being said, Aristotle saw the prime feature that made humans distinct from animals as rationality. We can think objectively, we know what true suffering and sorrow are, etc.
In looking at statements, Aristotle noticed that while the number of possible statements is unlimited, the way the subjects and predicates of propositions can be related to each other is definitely limited… (a) “Socrates is human”, (b) “Socrates is able to laugh”, and © “Socrates is pale” are Aristotle’s examples of these three types of relations. Humanity describes Socrates’ very essence. Paleness is completely unrelated to Socrates’ nature, and caused by him not getting enough sun.
So really, something is a person because it is a person in its own right (essence, nature). Someone cannot “gain” personhood, they either have it or they don’t. I cannot be a person one moment and cease to be one in the next. Therefore, being a person is not an attribute, it is the very concept of being for the particular thing. This is demonstrated here:
Finally, while Socrates’ ability to laugh does not define his essential nature, it is caused by his very humanity. This can be seen if we ask why Socrates, or anyone, can laugh. This can be seen if we ask why Socrates, or anyone, can laugh. To do that, you must perceive the situation and judge how odd it is compared to the norm. Then you laugh (or cry). This requires perception (animality) and thought (rationality). Thus, the cause of Socrates’ ability to laugh is his nature as a rational animal.
So a rational animal must be able to objectively, of its own will and not by reflex, cry or laugh as given examples. Those abilities cannot exist of their own right, but must be ultimately subject to the control of an essence or nature. Socrates’ nature is human, and because of that, he laugh. It is not the other way around. [This is the source for every quote above.]
Then comes the bigger question: now that we know of humans’ animality, what is a rational animal?
Man is the only rational animal. Animals are instinct (unless if they are influenced by us, like by domestication or through animalistic attachment, as with a house pet). Real humans are rationality. We control ourselves, our motions and thoughts. They are dependent on us being humans and come from us being humans, if that makes sense.
New Advent’s Catholic Encyclopedia leaves us with some final thoughts, a definition summarizing Aristotle’s ideas on humanity:
Man is a rational animal. This signifies no more than that, in the system of classification and definition shown in [Aristotle’s work], man is a substance, corporeal, living, sentient, and rational… It has been said that man’s animality is distinct in nature from his rationality, though they are inseparably joined, during life, in one common personality.
So we have it above:** man is an animal who possesses rationality. We control ourselves and our instincts are subject to us, not the other way around.** This is the very essence of the human.