man body is composed of these
numerous ideas of its component parts. Q.E.D.

PROP. 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 Coroll. of
Lemma iii.), wherefore their idea 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.

PROP. 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.
In other words, it will have the idea which does not exclude, but
postulates the existence or presence of the nature of the
external body; therefore the mind (by II. xvi., Coroll. 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.

Proof.--When external bodies determine the fluid parts of the
human body, so that they often impinge on the softer parts, they
change the surface of the last named (Post. v.); hence (Ax. ii.,
after the Coroll. of Lemma iii.) they are refracted therefrom in
a different manner from that