Hence it follows that man is composed of mind and body, and that the human body exists as we perceive it.
Hence we see not only that the human mind is united to the body, but also what is to be understood by the union of the mind and body. But no one can understand it adequately or distinctly without knowing adequately beforehand the nature of our body; for those things which we have proved hitherto are altogether general, nor do they refer more to man than to other individuals, all of which are animate, although in different degrees. For of everything there necessarily exists in God an idea of which He is the cause, in the same way as the idea of the human body exists in Him; and therefore everything that we have said of the idea of the human body is necessarily true of the idea of any other thing. We cannot, however, deny that ideas, like objects themselves, differ from one another, and that one is more excellent and contains more reality than another, just as the object of one idea is more excellent and contains more reality than another. Therefore, in order to determine the differences between the human mind and other things and its superiority over them, we must first know, as we have said, the nature of its object, that is to say, the nature of the human body. I am not able to explain it here, nor is such an explanation necessary for what I wish to demonstrate.
This much, nevertheless, I will say generally, that in proportion as one body is better adapted than another to do or suffer many things, in the same proportion will the mind at the same time be better adapted to perceive many things, and the more the actions of a body depend upon itself alone, and the less other bodies coöperate with it in action, the better adapted will the mind be for distinctly understanding. We can thus determine the superiority of one mind to another; we can also see the reason why we have only a very confused knowledge of our body, together with many other things which I shall deduce in what follows.
The Complexity of the Human Mind
The idea which constitutes the formal Being of the human mind is the idea of a body which is composed of a number of individuals composite to a high degree. But an idea of each individual composing the body must necessarily exist in God; therefore the idea of the human body is composed of these several ideas of the component parts. The idea which constitutes the formal Being of the human mind is not simple, but is composed of a number of ideas.
All ways in which any body is affected follow at the same time from the nature of the affected body, and from the nature of the affecting body; therefore the idea of these modifications necessarily involves the nature of each body, and therefore the idea of each way in which the human body is affected by an external body involves the nature of the human body and of the external body.
Hence it follows, in the first place, that the human mind perceives the nature of many bodies together with that of its own body.
It follows, secondly, that the ideas we have of external bodies indicate the constitution of our own body rather than the nature of external bodies.
Imagination
If the human body be affected in a way which involves the nature of any external body, the human mind will contemplate that external body as actually existing or as present, until the human body be affected by a mode which excludes the existence or presence of the external body.