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. 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.