Note.- This proposition is more clearly understood from II:xvi.Coroll.ii. For imagination is an idea, which indicates rather the present disposition of the human body than the nature of the external body; not indeed distinctly, but confusedly; whence it comes to pass, that the mind is said to err. For instance, when we look at the sun, we conceive that it is distant from us about two hundred feet; in this judgment we err, so long as we are in ignorance of its true distance; when its true distance is known, the error is removed, but not the imagination; or, in other words, the idea of the sun, which only explains tho nature of that luminary, in so far as the body is affected thereby: wherefore, though we know the real distance, we shall still nevertheless imagine the sun to be near us. For, as we said in III:xxxv.note, we do not imagine the sun to be so near us, because we are ignorant of its true distance, but because the mind conceives the magnitude of the sun to the extent that the body is affected thereby. Thus, when the rays of the sun falling on the surface of water are reflected into our eyes, we imagine the sun as if it were in the water, though we are aware of its real position; and similarly other imaginations, wherein the mind is deceived whether they indicate the natural disposition of the body, or that its power of activity is increased or diminished, are not contrary to the truth, and do not vanish at its presence. It happens indeed that, when we mistakenly fear an evil, the fear vanishes when we hear the true tidings; but the contrary also happens, namely, that we fear an evil which will certainly come, and our fear vanishes when we hear false tidings; thus imaginations do not vanish at the presence of the truth, in virtue of its being true, but because other imaginations, stronger than the first, supervene and exclude the present existence of that which we imagined, as I have shown in II:.xvii. Prop. II. We are only passive, in so far as we are apart of Nature, which cannot be conceived by itself without other parts. Proof.- We are said to be passive, when something arises in us, whereof we are only a partial cause (III:Def.ii.), that is (III:Def.i.), something which cannot be deduced solely from the laws of our nature. We are passive therefore in so far as we are a part of Nature, which cannot be conceived by itself without other parts. Q.E.D. Prop. III. The force whereby a man persists in existing is limited, and is infinitely surpassed by the power of external causes. Proof.-This is evident from the axiom of this part. For, when man is given, there is something else - say A - more powerful; when A is given, there is something else - say B - more powerful than A, and so on to infinity; thus the power of man is limited by the power of some other thing, and is infinitely surpassed by the power of external causes. Q.E.D.