Originally posted here.
Introduction:
Throughout the centuries the Christian tradition has had a love-hate  relationship with Aristotle. His logic and metaphysical categories  provided strong tools for developing and formalizing such classic  doctrines as the Incarnation and the Holy Trinity. On the other hand, he  famously held that the universe was eternal and that there was no first  man such as Adam, putting him at odds with Christian belief. Yet in the  midst of this conflict, there is much to be gathered and adopted by the  Christian. Here I’d like to examine a few of Aristotle’s ideas about  the essences of things and see how they relate to the Christian view of  man.
Essences:
The first thing to note is that humans, along with every other kind of  thing, have “essences” or “natures”. An essence is simply that in virtue  of which a thing is what it is. It’s what makes the definition of a  thing true. It’s also the grounding of a thing’s essential properties,  i.e. those properties which are intrinsic to that type of thing. Without  essences we could not form inductive laws, we could not differentiate  between kinds of things, and we couldn’t even properly define our  concepts.
Essences entail that things have natural ends or functions (what  Aristotle calls the “telos”). It would be incorrect to understand  “function” in terms of the functions of human artifacts such as wheels  or fishing rods. Rather, things have functions in the sense that they function in a certain natural way.  So we can see clearly that by their very essences things like dogs or  snakes have natural functions, including sensory perception,  reproduction, self-change, and sustenance. This is what makes it good  for them to pursue certain goals, like eating food or reproducing. To  generalize, it is good for them to fulfill their functions.
This may seem irrelevant, but it has truly significant consequences  for us as well. It gives medicine a normative structure, making sense of  the strong and reasonable intuition that things such as broken bones or  tumors are defects, while other things such as firmness of muscle or  thorough digestion are indicators of health. It means that we all have  certain functions which it is objectively good for us to fulfill. This  can provide a foundation for objective morality and a guide to living  our lives. Thus understood, essences are both indispensable and  significant.
The Christian Picture:
So what is essential to humanity? Well, recall that essences are what  differentiate things from entities of other kinds. In attempting to  discover our essence then, it would be useful to see how we differ from  other things. We share some characteristics with non-living things  because we too are made of matter and have mass. But we are different  from them precisely in virtue of the fact that we are alive. We are  different from some living things like plants due to our sensory  capabilities which we have in common with other animals. Yet we humans  can be distinguished even from these creatures by our rationality.  Unlike other animals, we can abstract from concrete particulars to  general universal truths. Thus, humans are by their very essence rational animals.
How well does this fit into the Christian view of things? Consider  the idea that God is a being with infinite knowledge, and thus an  intellect. In Romans 11:33 for instance, Paul the Apostle praises God’s  omniscience: ” O the depth of the riches of the wisdom and of the  knowledge of God!” Aristotle’s view confirms precisely what the believer  says, that is, that humans are by their very essence endowed with an  intellect and will like God’s. As Genesis 1:27 says: “God  created man  to his own image: to the image of God he created him: male  and female  he created them.” Every human, male and female, is essentially made in  God’s image.
We can also conclude from Aristotle’s view that, because humans have  an essence with natural ends, there are some things that are objectively  good for all of us, and that these objective moral truths are available  to anyone who uses his or her reason and conscience. This confirms what  we find in Romans 2:14-15: “For when the Gentiles, who have not the  law, do by nature those things that are of the law; these, having not  the law, are a law to themselves. Who show the work of the law written  in their hearts, their conscience bearing witness to them.”
Much more could be said about all of these ideas. Yet it becomes  clear how strongly these Aristotelian theses can be used to support what  the Christian affirms. Hence, there is much fruit to be found by the  Christian in the Aristotelian tradition, making the ancients very worthy  of our attention.