1. By a thing which is causa sui, its own cause, I mean a thing the essence of which involves the existence of it, or a thing which cannot be conceived except as existing.
2. I call a thing finite, suo genere, when it can be limited by another (or others) of the same nature—e.g. a given body is called finite, because we can always conceive another body enveloping it; but body is not limited by thought, nor thought by body.
3. By substance I mean what exists in itself and is conceived by itself; the conception of which, that is, does not involve the conception of anything else as the cause of it.
4. By attribute I mean whatever the intellect perceives of substance as constituting the essence of substance.
5. Mode is an affection of substance, or is that which is in something else, by and through which it is conceived.
6. God is a being absolutely infinite; a substance consisting of infinite attributes, each of which expresses his eternal and infinite essence.
EXPLANATION.
I say absolutely infinite, not infinite suo genere—for of what is infinite suo genere only, the attributes are not infinite but finite; whereas what is infinite absolutely contains in its own essence everything by which substance can be expressed, and which involves no impossibility.
7. That thing is 'free' which exists by the sole necessity of its own nature, and is determined in its operation by itself only. That is 'not free' which is called into existence by something else, and is determined in its operation according to a fixed and definite method.
8. Eternity is existence itself, conceived as following necessarily and solely from the definition of the thing which is eternal.