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. EXPLANATION. Because existence of this kind is conceived as an eternal verity, and, therefore, cannot be explained by duration, even though the duration be without beginning or end. So far the definitions; then follow the AXIOMS. 1. All things that exist, exist either of themselves or in virtue of something else. 2. What we cannot conceive of as existing in virtue of something else, we must conceive through and in itself. 3. From a given cause an effect necessarily follows, and if there be no given cause no effect can follow. 4. Things which have nothing in common with each other cannot be understood through one another--_i.e._ the conception of one does not involve the conception of the other. 5. To understand an effect implies that we understand the cause of it. 6. A true idea is one which corresponds with its _ideate_. 7. The essence of anything which can be conceived as non-existent does not involve existence. Such is our metaphysical outfit of simple ideas with which to start upon our enterprise of learning. The larger number of them, so far from being simple, must be absolutely without meaning to persons whose minds are undisciplined in metaphysical abstraction; they become only intelligible propositions as we look back upon them with the light of the system which they are supposed to contain.