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 the human body. Proof.--Thought is an attribute of God (II. i.); therefore (II. iii.) there must necessarily be in God the idea both of thought itself and of all its modifications, consequently also of the human mind (II. xi.). Further, this idea or knowledge of the mind does not follow from God, in so far as he is infinite, but in so far as he is affected by another idea of an individual thing (II. ix.). But (II. vii.) the order and connection of ideas is the same as the order and connection of causes; therefore this idea or knowledge of the mind is in God and is referred to God, in the same manner as the idea or knowledge of the body. Q.E.D. PROP. XXI. This idea of the mind is united to the mind in the same way as the mind is united to the body. Proof.--That the mind is united to the body we have shown from the fact, that the body is the object of the mind (II. xii. and xiii.); and so for the same reason the idea of the mind must be united with its object, that is, with the mind in the same manner as the mind is united to the body. Q.E.D.