I
The images of things are modifications of the human body, and the ideas of these modifications represent to us external bodies as if they were present, that is to say, these ideas involve both the nature of our own body and at the same time the present nature of the external body. If, therefore, the nature of the external body be like that of our body, then the idea of the external body which we imagine will involve a modification of our body like that of the external body. Therefore, if we imagine any one who is like ourselves to be affected by a modification, this imagination will express a modification of our body like that modification, and therefore we shall be modified with a similar modification ourselves, because we imagine something like us to be modified with the same. If, on the other hand, we hate a thing which is like ourselves, we shall so far be modified by a modification contrary and not similar to that with which it is modified.
If we imagine that a person enjoys a thing, that will be a sufficient reason for making us love the thing and desiring to enjoy it. If we imagine that a person enjoys a thing which only one can possess, we do all we can to prevent his possessing it. His enjoyment of the thing is an obstacle to our joy, and we endeavor to bring into existence everything which we imagine conduces to joy, and to remove or destroy everything opposed to it, or which we imagine conduces to sorrow.
We see, therefore, that the nature of man is generally constituted so as to pity those who are in adversity and envy those who are in prosperity, and he envies with a hatred which is the greater in proportion as he loves what he imagines another possesses. We see also that from the same property of human nature from which it follows that men pity one another it also follows that they are envious and ambitious. If we will consult experience, we shall find that she teaches the same doctrine, especially if we consider the first years of our life. For we find that children, because their body is, as it were, continually in equilibrium, laugh and cry merely because they see others do the same; whatever else they see others do they immediately wish to imitate; everything which they think is pleasing to other people they want. And the reason is, as we have said, that the images of things are the modifications themselves of the human body, or the ways in which it is modified by external causes and disposed to this or that action.