[quote="utunumsint, post:5, topic:334834"]
Essence is, as Empter said, is what makes a thing what it is. It is what distinguishes one thing from another thing.
The essence of human beings is humanity. Humanity is distinguished from the essence of animals in that the definition of what it means to be a human being includes rationality.
The essence of a triangle has three points.
The essence of a square has four points and, although it includes all the three points of a triangle, it is essentially different from a triangle because of that fourth point.
An essence is what must belong to a substance for it to remain that substance. For example, if I take a square and remove its fourth point and one of its lines, then join up the third line with the second point, there has been a substantial change in the form of the square so that it can no longer be considered a square, but must now be considered a triangle.
As you may have noticed, I have just brought in the concept of a substance. Substances can be composites of form and matter, or form alone. For composite substances, the matter is what designates one instance of an essence from another instance of an essence. For example, I share all it means to be a human being with you, but my designate matter is different. Not only that, I have a unique rational soul (that cannot be reduced to matter, but I'll come to that later). One Apple tree may share the same nature as another apple tree, but is differentiated from each other by their designate matter. Or, one triangle may share all the same essential qualities of the essence of a triangle (e.g. triangularity) but one may be much larger than another.
Here you will notice I introduced the concept of size. That you may be taller than I am makes no difference to the fact that we both share the same essence. Tallness, fatness, the fact I have brown hair, the fact that you have red hair, the fact that I am in this location as opposed to that location, all of these are called accidents. They exist only on account of being associated to a specific substance. Scientists only study designate matter, and all we can see from designate matter is its accidents.
Aristotle came up with ten categories by which you could categorize all things that exist. The first category is that of substance. It defines the essence of a thing on account of which that thing falls into a specific nature according to genera and species. The other nine categories all have to do with accidents that accrue in different ways in a substance.
Essence is an abstract concept. Abstraction is something only rational creatures can do. When we are born, Aristotle and Aquinas, believed that our minds are blank slates (tabula rasa). When we start to grow, we start to observer our environment and the substances around us. We see things, and start to group them according to the ten categories. For example, I can imagine, as a baby, that the square I am gumming and feeling with my hands has the exact same shape of the much larger square across the room. The moment the concept of squareness comes into my baby mind is the moment my rational nature starts to assert itself. I have created a universal in my mind. A essential truth about the nature of all squares. The more I grow, the more I grasp the essential and accidental nature of things. Here we move out of the world of sensible forms and into the world of intelligible forms.
It is at this point that Aristotle and Aquinas can make an case for the immateriality of the soul. Because the universal concept of square can exist only in the mind, but it must be something, and the truth that a square must have four sides and four points, is always true, then it must be something real and eternal. If we can apprehend something of this nature, something real and eternal, then our rational nature must also share in that immateriality.
I hope this helps,
This is good! :thumbsup: