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.