Ethics, Part V, Proposition 29

In so far as the mind conceives the present existence of its body, it to that extent conceives duration which can be determined by time, and to that extent only has it the power of conceiving things in relation to time (V. xxi. II. xxvi.). But eternity cannot be explained in terms of duration (I. Def. viii. and explanation). Therefore to this extent the mind has not the power of conceiving things under the form of eternity, but it possesses such power, because it is of the nature of reason to conceive things under the form of eternity (II. xliv. Coroll. ii.), and also because it is of the nature of the mind to conceive the essence of the body under the form of eternity (V. xxiii.), for besides these two there is nothing which belongs to the essence of mind (II. xiii.). Therefore this power of conceiving things under the form of eternity only belongs to the mind in virtue of the mind's conceiving the essence of the body under the form of eternity. Q.E.D.

Things are conceived by us as actual in two ways; either as existing in relation to a given time and place, or as contained in God and following from the necessity of the divine nature. Whatsoever we conceive in this second way as true or real, we conceive under the form of eternity, and their ideas involve the eternal and infinite essence of God, as we showed in II. xlv. and note, which see.