I'm in my last year of a philosophy degree, so it's good to have a
question that places me in my element! I'll begin with some quotes
from Aquinas on essence. Following this will be notes from various
universities on Aquinas, and at the end will be my direct summarial
answer of your question.
On Being and Essence (DE ENTE Et ESSENTIA) Translation © 1997 by
Robert T. Miller
"...essence signifies something common to all natures through which
the various beings are placed in the various genera and species, as
humanity is the essence of man, and so on".
"Essence is also called form, for the certitude of every thing is
signified through its form, as Avicenna says in his Metaphysicae I,
cap. 6. The same thing is also called nature, taking nature in the
first of the four senses that Boethius distinguishes in his book De
Persona et Duabus Naturis cap. 1 (PL 64, 1341B), in the sense, in
other words, that nature is what we call everything that can in any
way be captured by the intellect, for a thing is not intelligible
except through its definition and essence".
"In composite substances we find form and matter, as in man there are
soul and body. We cannot say, however, that either of these is the
essence of the thing. That matter alone is not the essence of the
thing is clear, for it is through its essence that a thing is knowable
and is placed in a species or genus. But matter is not a principle of
cognition; nor is anything determined to a genus or species according
to its matter but rather according to what something is in act. Nor is
form alone the essence of a composite thing, however much certain
people may try to assert this. From what has been said, it is clear
that the essence is that which is signified by the definition of the
"Therefore, the essence clearly comprises both matter and form"
The Columbia Encyclopedia, Sixth Edition. 2001.
Thomas Aquinas, Saint
"The 13th cent. was a critical period in Christian thought, which was
torn between the claims of the Averroists and Augustinians. Thomas
opposed both schools, the Averroists led by Siger de Brabant, who
would separate faith and truth absolutely, and the Augustinians, who
would make truth a matter of faith. St. Thomas held that reason and
faith constitute two harmonious realms in which the truths of faith
complement those of reason; both are gifts of God, but reason has an
autonomy of its own. Thus he vindicated Aristotle against those who
saw him as the inspiration of AverroŽs and heresy.
The first principle of philosophy according to St. Thomas is the
affirmation of being. From this he proceeded to a consideration of the
manner in which the intellect achieves knowledge. For humans all
knowledge begins by way of the senses, which are the medium through
which he grasps the intelligible world, the universal. According to
the position of Thomas, which is known as moderate realism, the form
or the universal may be said to exist in three ways: in God, in
things, and in the mind (see universals). He argues that it is by the
knowledge of things that we come to know of Gods existence. In the
natural order what God is can be known only by analogy and negation.
Thomass conviction that the existence of God can be discovered by
reason is shown by his proofs of the existence of God. His metaphysics
relies on the Aristotelian concepts of potency and act, matter and
form, being and essence. A thing that requires completion by another
is said to be in potency to that other; the realization of potency is
called actuality. The universe is conceived of as a series of things
arranged in an ascending order of potency, an act at once crowned and
created by God, who alone is pure act. Two other pairs of metaphysical
conceptsmatter and form, essence and beingare special cases of
potency and act. St. Thomass moral philosophy is derived from these
distinctions as well, since the opposite of being does not exist and
since the good is identical with being, evil is but the absence of
Saint Thomas Aquinas
"It is evident that material substances exist contingently. They come
into being and they pass out of being and while they exist, existence
is not what they are. Thomas accepts from Boethius that it is
self-evident that what a thing is and for it to exist differ (diversum
est esse et id quod est). Material things depend upon causes to exist,
both to become and to be. There is no need to dwell on this except
insofar as it provides a springboard to speak of immaterial substance.
Only in God is it the case that what he is and that he is are
identical: God is existence. The phrase Thomas uses to express this is
ipsum esse subsistens. Of course this is paradoxical. Existence is the
actuality of a substance, not itself something subsistent. This is
true with material substances, but when we ask what we mean by saying
that God exists, we have to negate aspects of material existence in
order to avoid speaking of Him as if he were a contingent being.
The problem that Thomas now faces is how to speak of the immaterial
substances which are less than God although superior to material
substances, that is, angels. For a material thing to exist is for its
form actually to inhere in its matter. But what is it for a pure form
to exist? Since immaterial substances less than God are dependent on
the divine causality in order to exist, existence cannot be what they
are, of their essence. In short, in angels too there is a distinction
of essence and existence. Thomas notes that a created separate
substance is what it is and not another thing: that is, it has the
perfection it has, but not unlimited perfection. It is a being of a
kind, not being as such. Gabriel is perfect as to Gabrielitas, but he
is not Raphael or Michael. Form thus operates as a restriction on
existence as such. In God alone is there unrestricted existence; he is
existence, ipsum esse subsistens."
The Metaphysics of St. Thomas in one easy but not simple lesson
( You think we are kidding, right?)
"The metaphysics of St. Thomas is based on two central principles:
essence and existence. And it is the relationship between essence and
existence that is the key to the metaphysics of St. Thomas.
All around us are different kinds of things: apples, butterflies, and
elephants. And we dont confuse one with the other. What is essence?
It is what a thing is. It is what makes a thing to be what it is. It
is its whatness.
All around us are existing things. We have no doubt there are apples,
butterflies, and elephants. They are certainly different, but they all
exist. They are. Existence is the thatness of things in the sense of
the very fact that they exist, or the isness of things.
Neither essence nor existence is hard to grasp, for both of these
ideas emerge from our experience. They are two fundamental ways in
which we can look at things. We can ask about the elephant, what is
it? And we can assert about the elephant that it is. But we have to go
deeper and explore the relationship between essence and existence.
One way to do this is to ask ourselves what makes a what to be a what,
or what is whatness? What is the essence of essences? These are
strange questions, to be sure, but ones that Thomas posed in his own
way, and to which he gave a fascinating answer that was to
An essence, or a what, is a certain capacity for existence. Different
essences or whats are partial reflections or refractions of what it
means to exist, to be, just like different colors of the rainbow are
partial refractions of sunlight."
Basically: Aquinas was a Catholic philosopher who believed that the
existence of God and the practice of Philosophy did not have to be
Existence: is what happens when something exists. For example, when I
am born, I exist, and when I die, I cease to exist. Things need causes
in order to exist - if I do not make a sandwich, no sandwich exists.
The fact that something exists does not prove what it *is*.
Essence: What something *is*. For example, there is something
essential to humans that makes us human and not anything else (our
genetic make up, although this is definitely not an example Aquinas
would have used.) The fact that something has an essence does not mean
that it exists. For example, we know what the essence of a unicorn is
- it is horse-like, but with a horn on its forehead and wings - but
this does not mean that a unicorn exists.
Aquinas makes the distinction between essence and existence because it
seems self-evident to him that 'what something is' and 'whether that
thing exists' are entirely different. Aquinas' moral philosophy comes
from this distinction as well. He holds that 'the good' is the same as
'being', and since there is no absence of being (being just does not
exist), evil is then the absence of good. This is akin to the
Epicurean notion that the limit to pleasure is the absence of pain.
You might also want to look at Aristotle, as has been mentioned above
Aquinas is Aristotillian in his thinking.
Hope this has helped you, if you need clarification please ask,