P17: If the human Body is affected with a mode that involves the nature of an external body, the human Mind will regard the same external body as actually existing, or as present to it, until the Body is affected by an affect43 that excludes the existence or presence of that body.

Dem.: This is evident. For so long as the human Body is so affected, the human Mind (by P12) will regard this affection of the body, i.e. (by P16), it will have the idea of a mode that actually exists, an idea that involves the nature of the external body, i.e., an idea that does not exclude, but posits, the existence or presence of the nature of the external body. And so the Mind (by P16 C1) will regard the external body as actually existing, or as present, until it is affected, etc., q.e.d.

Cor.: Although the external bodies by which the human body has once been affected neither exist nor are present, the mind will still be able to regard them as if they were present.

Dem.: While external bodies so determine the fluid parts of the human body that they often thrust against the softer parts, they change (by Post. 5) their surfaces with the result (see A2” after L3) that they are reflected from it in another way than they used to be before, and still later, when the fluid parts, by their spontaneous motion, encounter those new surfaces, they are reflected in the same way as when they were driven against those surfaces by the external bodies. Consequently, while, thus reflected, they continue to move, they will affect the human Body with the same mode, concerning which the Mind (by P12) will think again, i.e. (by P17), the Mind will again regard the external body as present; this will happen as often as the fluid parts of the human body encounter the same surfaces by their spontaneous motion. So although the external bodies by which the human Body has once been affected do not exist, the Mind will still regard them as present, as often as this action of the body is repeated, q.e.d.

Schol.: We see, therefore, how it can happen (as it often does) that we regard as present things that do not exist. This can happen from other causes also, but it is sufficient for me here to have shown one through which I can explain it as if I had shown it through its true cause; still, I do not believe that I wander far from the true {cause} since all those postulates which I have assumed contain hardly anything that is not established by experience which we cannot doubt, after we have shown that the human Body exists as we are aware of it (see P13 C). Furthermore (from P17 C and P16 C), we clearly understand what is the difference between the idea of, say, Peter, which constitutes the essence of Peter’s mind, and the idea of Peter which is in another man, say in Paul. For the former directly explains the essence of Peter’s body, and does not involve existence, except so long as Peter exists; but the latter indicates the condition of Paul’s body more than Peter’s nature {NS: see P16 C2}, and therefore, while that condition of Paul’s body lasts, Paul’s Mind will still regard Peter as present to itself, even though Peter does not exist.

Next, to retain the customary words, the affections of the human Body whose ideas present external bodies as present to us, we shall call images of things, even if they do not reproduce the {NS: external} figures of things. And when the Mind regards bodies in this way, we shall say that it imagines.

And here, in order to begin to indicate what error is, I should like you to note that the imaginations of the Mind, considered in themselves contain no error, or that the Mind does not err from the fact that it imagines, but only insofar as it is considered to lack an idea that excludes the existence of those things that it imagines to be present to it. For if the Mind, while it imagined nonexistent things as present to it, at the same time knew that those things did not exist, it would, of course, attribute this power of imagining to a virtue of its nature, not to a vice—especially if this faculty of imagining depended only on its own nature, i.e. (by ID7), if the Mind’s faculty of imagining were free.