Proof.- 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. Note.- 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.&Note, which see. Prop. XXX. Our mind, in so far as it knows itself and the body under the form of eternity, has to that extent necessarily a knowledge of God, and knows that it is in God, and is conceived through God. Proof.- Eternity is the very essence of God, in so far as this involves necessary existence (I:Def.viii.). Therefore to conceive things under the form of eternity, is to conceive things in so far as they are conceived through thp essence of God as real entities, or in so far as they involve existence through the essence of God; wherefore our mind, in so far as it conceives itself and the body under the form of eternity, has to that extent necessarily a knowledge of God, and knows, &c. Q.E.D. Prop. XXXI. The third kind of knowledge depends on the mind, as its formal cause, in so far as the mind itself is eternal. Proof.- The mind does not conceive anything under the form of eternity, except in so far as it conceives its own body under the form of eternity (V:xxix.); that is, except in so far as it is eternal (V:xxi., V:xxiii.); therefore (by the last Prop.), in so far as it is eternal, it possesses the knowledge of God, which knowledge is necessarily adequate (II:xlvi.); hence the mind, in so far as it is eternal, is capable of knowing everything which can follow from this given knowledge of God (II:xl.), in other words, of knowing things by the third kind of knowledge (see Def. in II:xl.Note.ii.), whereof accordingly the mind (III:Def.i.), in so far as it is eternal, is the adequate or formal cause of such knowledge. Q.E.D.