e, from the thought of the word pomum (an apple), a
Roman would straightway arrive at the thought of the fruit apple,
which has no similitude with the articulate sound in question,
nor anything in common with it, except that the body of the man
has often been affected by these two things; that is, that the
man has often heard the word pomum, while he was looking at the
fruit; similarly every man will go on from one thought to
another, according as his habit has ordered the images of things
in his body. For a soldier, for instance, when he sees the
tracks of a horse in sand, will at once pass from the thought of
a horse to the thought of a horseman, and thence to the thought
of war, &c.; while a countryman will proceed from the thought of
a horse to the thought of a plough, a field, &c. Thus every man
will follow this or that train of thought, according as he has
been in the habit of conjoining and associating the mental images
of things in this or that manner.

PROP. XIX. The human mind has no knowledge of the body, and does
not know it to exist, save through the ideas of the modifications
whereby the body is affected.

Proof.--The human mind is the very idea or knowledge of the
human body (II. xiii.), which (II. ix.) is in God, in so far as
he is regarded as affected by another idea of a particular thing
actually existing: or, inasmuch as (Post. iv.) the human body
stands in need of very many bodies whereby it is, as it were,
continually regenerated; and the order and connection of ideas
is the same as the order and connection of causes (II. vii.);
this idea will therefore be in God, in so far as he is regarded
as affected by the ideas of very many particular things. Thus
God has the idea of the human body, or knows the human body, in
so far as he is affected by very many other ideas, and not in so
far as he constitutes the nature of the human mind; that is (by
II. xi. Coroll.), the human mind does not know the human body.
But the ideas of the modifications of body are in God, in so far
as he constitutes the nature of the human mind, or the human
mind perceives those modifications (II. xii.), and consequently
(II. xvi.) the human body itself, and as actually existing;
therefore the mind perceives thus far only the human body.
Q.E.D.

PROP. XX. The idea or knowledge of the human mind is also in
God, following in God in the same manner, and being referred to
God in the same manner, as the idea or knowledge of