Éthique, Partie V, Proposition 1

The order and connection of ideas is the same (II. vii.) as the order and connection of things, and vice versa the order and connection of things is the same (II. vi. Coroll. and vii.) as the order and connection of ideas. Wherefore, even as the order and connection of ideas in the mind takes place according to the order and association of modifications of the body (II. xviii.), so vice versa (III. ii.) the order and connection of modifications of the body takes place in accordance with the manner, in which thoughts and the ideas of things are arranged and associated in the mind. Q.E.D.