Seven important definitions emerge from a consideration of language and reality.

1 The essence is that which makes a being what it is and without which it would not be the kind of being it is.

2 Nature is essence viewed as the source of activity.

3 The individual is constituted of essence existent in quantified matter plus other accidents. Essence is that which makes the individual like other members of its class. Quantified matter is that which makes t he individual different from other individuals in its class because matter, extended by reason of its quantity, must be this or that matter, which by limiting the form individuates it. Accidents are those notes (shapes, color, weight , size, etc.) by which we perceive the difference between the individuals of a class. The individuals within a species (for example, all human beings) are essentially the same. But they are not merely accidentally different; t hey are individually different. Even if individuals were as alike as the matches in a box of matches or the pins in a paper of pins, they would be nonetheless individually different because the matter in one is not the matter in the other but is a different quantity or part even though of t he same kind and amount.

4 A percept is the sense-apprehension of an individual reality (in its presence).

5 A phantasm is the mental image of an individual reality (in its absence).

6 A general concept is the intellectual apprehension of essence.

7 An empirical concept is the indirect intellectual apprehension of an individual. The intellect can know individual objects only indirectly in the phantasms because individuals are material, with one exception, the int ellect itself; because it is a spiritual individual, the intellect can know itself directly and reflexively. (See Saint Thomas Aquinas, Summa Theologica, Part I, Question 86, Articles 1 and 3.)