When external bodies so determine the fluid parts of the human body that they often strike upon the softer parts, the fluid parts change the plane of the soft parts, and thence it happens that the fluid parts are reflected from the new planes in a direction different from that in which they used to be reflected, and that also afterwards when they strike against these new planes by their own spontaneous motion, they are reflected in the same way as when they were impelled towards those planes by external bodies. Consequently those fluid bodies produce a modification in the human body while they keep up this reflex motion similar to that produced by the presence of an external body. The mind, therefore, will think as before, that is to say, it will again contemplate the external body as present. This will happen as often as the fluid parts of the human body strike against those planes by their own spontaneous motion. Therefore, although the external bodies by which the human body was once affected do not exist the mind will perceive them as if they were present so often as this action is repeated in the body.
We see, therefore, how it is possible for us to contemplate things which do not exist as if they were actually present. This may indeed be produced by other causes, but I am satisfied with having here shown one cause through which I could explain it, just as if I had explained it through the true cause. I do not think, however, that I am far from the truth, since no postulate which I have assumed contains anything which is not confirmed by an experience that we cannot mistrust, after we have proved the existence of the human body as we perceive it.
We clearly see, moreover, what is the difference between the idea, for example, of Peter, which constitutes the essence of the mind itself of Peter, and the idea of Peter himself which is in another man; for example, in Paul. For the former directly manifests the essence of the body of Peter himself, nor does it involve existence unless so long as Peter exists; the latter, on the other hand, indicates rather the constitution of the body of Paul than the nature of Peter; and therefore so long as Paul's body exists with that constitution, so long will Paul's mind contemplate Peter as present, although he does not exist. But in order that we may retain the customary phraseology, we will give to those modifications of the human body, the ideas of which represent to us external bodies as if they were present, the name of images of things, although they do not actually reproduce the forms of the things. When the mind contemplates bodies in this way, we will say that it imagines. Here I wish it to be observed, in order that I may begin to show what error is, that these imaginations of the mind, regarded by themselves, contain no error, and that the mind is not in error because it imagines, but only in so far as it is considered as wanting in an idea which excludes the existence of those things which it imagines as present. For if the mind, when it imagines non-existent things to be present, could at the same time know that those things did not really exist, it would think its power of imagination to be a virtue of its nature and not a defect, especially if this faculty of imagining depended upon its own nature alone, that is to say, if this faculty of the mind were free.
Association of Ideas and Memory
If the human body has at any time been simultaneously affected by two or more bodies, whenever the mind afterwards imagines one of them, it will also remember the others.
We clearly understand by this what memory is. It is nothing else than a certain concatenation of ideas, involving the nature of things which are outside the human body, a concatenation which corresponds in the mind to the order and concatenation of the modifications of the human body. I say, firstly, that it is a concatenation of those ideas only which involve the nature of things which are outside the human body, and not of those ideas which explain the nature of those things, for there are in truth ideas of the modifications of the human body, which involve its nature as well as the nature of external bodies. I say, in the second place, that this concatenation takes place according to the order and concatenation of the modifications of the human body, that I may distinguish it from the concatenation of ideas which takes place according to the order of the intellect, and enables the mind to perceive things through their first causes, and is the same in all men.
Hence we can clearly understand how it is that the mind from the thought of one thing at once turns to the thought of another thing which is not in any way like the first. For example, from the thought of the word pomum a Roman immediately turned to the thought of the fruit, which has no resemblance to the articulate sound pomum, nor anything in common with it, excepting this, that the body of that man was often affected by the thing and the sound; that is to say, he often heard the word pomum when he saw the fruit. In this manner each person will turn from one thought to another according to the manner in which the habit of each has arranged the images of things in the body. The soldier, for instance, if he sees the footsteps of a horse in the sand, will immediately turn from the thought of a horse to the thought of a horseman, and so to the thought of war. The countryman, on the other hand, from the thought of a horse will turn to the thought of his plow, his field, etc.; and thus each person will turn from one thought to this or that thought, according to the manner in which he has been accustomed to connect and bind together the images of things in his mind.
FOOTNOTES:
[14] [Similarly, it can be demonstrated that] extension is an attribute of God, or God is an extended thing.