Definitions
I. By body, I understand a mode which expresses in a certain and determinate manner the essence of God in so far as He is considered as the thing extended.
II. I say that to the essence of anything pertains that, which being given, the thing itself is necessarily posited, and being taken away, the thing is necessarily taken; or, in other words, that, without which the thing can neither be nor be conceived, and which in its turn cannot be nor be conceived without the thing.
III. By idea, I understand a conception of the mind which the mind forms because it is a thinking thing.
Explanation.—I use the word conception rather than perception because the name perception seems to indicate that the mind is passive in its relation to the object. But the word conception seems to express the action of the mind.
IV. By adequate idea, I understand an idea which, in so far as it is considered in itself, without reference to the object, has all the properties or internal signs (denominationes intrinsecas) of a true idea.
Explanation.—I say internal, so as to exclude that which is external, the agreement, namely, of the idea with its object.
V. Duration is the indefinite continuation of existence.
Explanation.—I call it indefinite because it cannot be determined by the nature itself of the existing thing nor by the efficient cause, which necessarily posits the existence of the thing but does not take it away.
VI. By reality and perfection I understand the same thing.