P18: If the human Body has once been affected by two or more bodies at the same time, then when the Mind subsequently imagines one of them, it will immediately recollect the others also.

Dem.: The Mind (by P17 C) imagines a body because the human Body is affected and disposed as it was affected when certain of its parts were struck by the external body itself.

But (by hypothesis) the Body was then so disposed that the Mind imagined two {or more} bodies at once; therefore it will now also imagine two {or more} at once, and when the Mind imagines one, it will immediately recollect the other also, q.e.d.

Schol.: From this we clearly understand what Memory is. For it is nothing other than a certain connection of ideas involving the nature of things which are outside the human Body—a connection that is in the Mind according to the order and connection of the affections of the human Body.

I say, first, that the connection is only of those ideas that involve the nature of things which are outside the human Body, but not of the ideas that explain the nature of the same things. For they are really (by P16) ideas of affections of the human Body which involve both its nature and that of external bodies.

I say, second, that this connection happens according to the order and connection of the affections of the human Body in order to distinguish it from the connection of ideas which happens according to the order of the intellect, by which the Mind perceives things through their first causes, and which is the same in all men.

And from this we clearly understand why the Mind, from the thought of one thing, immediately passes to the thought of another, which has no likeness to the first: as, for example, from the thought of the word pomum a Roman will immediately pass to the thought of the fruit {viz. an apple}, which has no similarity to that articulate sound and nothing in common with it except that the Body of the same man has often been affected by these two {NS: at the same time}, i.e., that the man often heard the word pomum while he saw the fruit.

And in this way each of us will pass from one thought to another, as each one’s association has ordered the images of things in the body. For example, a soldier, having seen traces of a horse in the sand, will immediately pass from the thought of a horse to the thought of a horseman, and from that to the thought of war, etc. But a Farmer will pass from the thought of a horse to the thought of a plow, and then to that of a field, etc. And so each one, according as he has been accustomed to join and connect the images of things in this or that way, will pass from one thought to another.