XV. The idea, which constitutes the actual being of the human mind, is not simple, but compounded of a great number of ideas. >>>>>Proof--The idea constituting the actual being of the human mind is the idea of the body (II. xiii.), which (Post. i.) is composed of a great number of complex individual parts. But there is necessarily in God the idea of each individual part whereof the body is composed (II. viii. Cor.); therefore (II. vii.), the idea of the human body is composed of each of these numerous ideas of its component parts. Q.E.D. XVI. The idea of every mode, in which the human body is affected by external bodies, must involve the nature of the human body, and also the nature of the external body. >>>>>Proof--All the modes, in which any given body is affected, follow from the nature of the body affected, and also from the nature of the affecting body (by Ax. i., after the Cor. of Lemma iii.), wherefore their idea is also necessarily (by I, Ax. iv.) involves the nature of both bodies; therefore, the idea of every mode, in which the human body is affected by external bodies, involves the nature of the human body and of the external body. Q.E.D. <<<<<Corollary I.--Hence it follows, first, that the human mind perceives the nature of a variety of bodies, together with the nature of its own. <<<<<Corollary II.--It follows, secondly, that the ideas, which we have of external bodies, indicate rather the constitution of our own body than the nature of external bodies. I have amply illustrated this in the Appendix to Part I. XVII. If the human body is affected in a manner which involves the nature of any external body, the human mind will regard the said external body as actually existing, or as present to itself, until the human body be affected in such a way, as to exclude the existence or the presence of the said external body. >>>>>Proof--This proposition is self-evident, for so long as the human body continues to be thus affected, so long will the human mind (II. xii.) regard this modification of the body --that is (by the last Prop.), it will have the idea of the mode as actually existing, and this idea involves the nature of the external body; therefore the mind (by II. xvi., Cor. i.) will regard the external body as actually existing, until it is affected, &c. Q.E.D. <<<<<Corollary--The mind is able to regard as present external bodies, by which the human body has once been affected, even though they be no longer in existence or present.