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.
When I said that we are affected by the image of an object in the future or the past with the same emotion with which we should be affected if the object we imagined were actually present, I was careful to warn the reader that this was true in so far only as we attend to the image alone of the object itself, for the image is of the same nature whether we have imagined the object or not; but I have not denied that the image becomes weaker when we contemplate as present other objects which exclude the present existence of the future object.
The image of a past or future object, that is to say, of an object which we contemplate in relation to the past or future to the exclusion of the present, other things being equal, is weaker than the image of a present object, and consequently the emotion towards a future or past object, other things being equal, is weaker then than the emotion towards a present object.
The desire which springs from a knowledge of good and evil can be easily extinguished or restrained, in so far as this knowledge is connected with the future, by the desire of things which in the present are sweet.