Ethics, Part V, Proposition 23

There is necessarily in God a concept or idea, which expresses the essence of the human body (last Prop.), which, therefore, is necessarily something appertaining to the essence of the human mind (II. xiii.). But we have not assigned to the human mind any duration, definable by time, except in so far as it expresses the actual existence of the body, which is explained through duration, and may be defined by time--that is (II. viii. Coroll.), we do not assign to it duration, except while the body endures. Yet, as there is something, notwithstanding, which is conceived by a certain eternal necessity through the very essence of God (last Prop.); this something, which appertains to the essence of the mind, will necessarily be eternal. Q.E.D.

This idea, which expresses the essence of the body under the form of eternity, is, as we have said, a certain mode of thinking, which belongs to the essence of the mind, and is necessarily eternal. Yet it is not possible that we should remember that we existed before our body, for our body can bear no trace of such existence, neither can eternity be defined in terms of time, or have any relation to time. But, notwithstanding, we feel and know that we are eternal. For the mind feels those things that it conceives by understanding, no less than those things that it remembers. For the eyes of the mind, whereby it sees and observes things, are none other than proofs. Thus, although we do not remember that we existed before the body, yet we feel that our mind, in so far as it involves the essence of the body, under the form of eternity, is eternal, and that thus its existence cannot be defined in terms of time, or explained through duration. Thus our mind can only be said to endure, and its existence can only be defined by a fixed time, in so far as it involves the actual existence of the body. Thus far only has it the power of determining the existence of things by time, and conceiving them under the category of duration.