Thomistic Philosophy
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Thomistic Metaphysics
Essence and Existence

Note: Arguably, everything has an essence, hence the reason why what one thing is can be distinguished from what something else is (even regarding things that only exist in the mind, for such things would still have essences, since we can think about what they are)   Click to Expand ⤵

  1. Two Kinds of Essence   ⤵
    1. Concrete Essence (a.k.a. Physical Essence)
      The essence as it exists concretely in nature, independent of the mind's thinking (i.e. an essence as it actually exists)
      (e.g. the essence of a particular horse has concrete essence, whereas the essence of a unicorn does not)   ⤵
      1. Simple Concrete Essence (a.k.a. Separated Substance or Spiritual Essence)
        A thing whose essence consists only of form
        (e.g. angels)
      2. Composite Concrete Essence (a.k.a. Material Substance)
        A thing whose essence consists of both form and matter (which includes all material beings)
        (e.g. humans)

    2. Abstract Essence (a.k.a. Metaphysical Essence)
      The essence according to the manner in which it is conceived by the mind
      (e.g. both horses and unicorns have abstract essences, for the mind can think about what both are, whether or not either one actually exists in reality)   ⤵
      1. Four Properties of Abstract Essence
        (these properties also apply to "form" in general)
        1. Immutability (a.k.a. Unchangeableness)
          An essence itself cannot be changed (but it can be removed/replaced by another)
        2. Indivisibility
          An essence itself cannot be divided (but it can be removed/replaced by others)
        3. [Conditional] Necessity
          An essence is always necessarily itself (that is, it necessarily exists if it exists)
        4. [Ideal] Eternity (a.k.a. Infinity or [Negative] Eternity)
          An essence is not confined to a particular number of things with that essence

  2. Synonyms of Essence
    (i.e. these terms are often interchangeable with "essence" but not always)
    1. Nature
      Essence expressed as how a thing tends to operate because of its essence
    2. Form
      Essence expressed as a thing's form (which is the primary part even of a composite essence, even though it is composed both matter and form)
      (e.g. the essence of a chair is primarily about its form [the way it is shaped for the purpose of sitting] rather than any particular matter it may be composed of [whether it be wood, metal, plastic, etc.])
    3. Species (a.k.a. Logical Definition)
      Essence expressed as comparing and contrasting it with other kinds of essences (i.e. its genus and specific difference)
      (e.g. the essence of a human can be expressed as the species of rational animal, with "animal" being the genus [the broader group it falls under] and "rational" being its specific difference [what distinguishes it from everything else under that genus])
    4. Quiddity (a.k.a. Whatness)
      Essence expressed as the answer to the question "What is this thing?"
    5. Substance
      Essence expressed as a thing in itself as distinct from any accidents it may have
      Note: For this reason, one might often hear "essence and accidents" distinguished from each other just as "substance and accidents" are, yet one can still speak of the "essence of accidents" when looking at accidents themselves (for when you talk about what some accident is, you are talking about its essence)
      (e.g. the essence of a human being [which is a substance] is distinct from any particular accidents that any particular human being might have, such as one's height, skin color, location in the world, etc.)

EXISTENCE : THE ACTUALITY OF ESSENCE (i.e. a thing's existence, as opposed to what the thing is)
Note: "Existence" here refers more narrowly to "actual existence," which is existence properly so-called, rather than the broader concept of "possible existence"
(e.g. the essence of a horse has existence because there are horses [i.e. at least one], whereas the essence of a unicorn does not exist because there are no unicorns)   Click to Expand ⤵
  1. Subsistence
    The way in which the essence of a substance actually exists (i.e. to exist "in itself" or "by itself")
  2. Inherence (a.k.a. Inhesion)
    The way in which the essence of an accident actually exists