An emotion is an idea by which the mind affirms a greater or less power of existence for the body than it possessed before, and therefore this idea has nothing positive which can be removed by the presence of the truth, and consequently the true knowledge of good and evil, in so far as it is true, can restrain no emotion. But in so far as it is an emotion will it restrain any other emotion, provided that the latter be the weaker of the two. From the true knowledge of good and evil, in so far as this is an emotion, necessarily arises desire, which is greater in proportion as the emotion from which it springs is greater. But this desire (by hypothesis), because it springs from our understanding, something truly follows therefore in us in so far as we act, and therefore must be understood through our essence alone, and consequently its strength and increase must be limited by human power alone. But the desires which spring from the emotions by which we are agitated are greater as the emotions themselves are greater, and therefore their strength and increase must be limited by the power of external causes, a power which, if it be compared with our own, indefinitely surpasses it. The desires, therefore, which take their origin from such emotions as these may be much stronger than that which takes its origin from a true knowledge of good and evil, and the former may be able to restrain and extinguish the latter. Desire is the very essence of man, that is to say, the effort by which a man strives to persevere in his being. The desire, therefore, which springs from joy, by that very emotion of joy is assisted or increased, while that which springs from sorrow, by that very emotion of sorrow is lessened or restrained, and so the force of the desire which springs from joy must be limited by human power, together with the power of an external cause, while that which springs from sorrow must be limited by human power alone. The latter is, therefore, weaker than the former. _How the Strength of the Emotions Varies_ I The imagination is an idea by which the mind contemplates an object as present, an idea which nevertheless indicates the constitution of the human body rather than the nature of the external object. Imagination, therefore, is an emotion in so far as it indicates the constitution of the body. But the imagination increases in intensity in proportion as we imagine nothing which excludes the present existence of the external object. If, therefore, we imagine the cause of an emotion to be actually present with us, that emotion will be intenser or stronger than if we imagined the cause not to be present.