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.
II
In so far as we imagine any object to be necessary do we affirm its existence, and, on the other hand, we deny its existence in so far as we imagine it to be not necessary and therefore the emotion towards an object which we imagine as necessary, other things being equal, is stronger than that towards an object that is possible, contingent, or not necessary.
In so far as we imagine an object as contingent, we are not affected by the image of any other object which posits the existence of the first, but, on the contrary (by hypothesis), we imagine some things which exclude its present existence. But in so far as we imagine any object in the future to be possible do we imagine some things which posit its existence, that is to say, things which foster hope or fear, and therefore the emotion towards an object which we know does not exist in the present, and which we imagine as possible, other things being equal, is stronger than the emotion towards a contingent object.
The emotion towards an object which we imagine to exist in the present is stronger than if we imagined it as future, and is much stronger if we imagine the future to be at a great distance from the present time. The emotion, therefore, towards an object which we imagine will not exist for a long time is so much feebler than if we imagined it as present, and nevertheless is stronger than if we imagined it as contingent; and therefore the emotion towards a contingent object is much feebler than if we imagined the object to be present to us.
In so far as we imagine an object as contingent, we are affected with no image of any other object which posits the existence of the first. On the contrary, we imagine (by hypothesis) certain things which exclude its present existence. But in so far as we imagine it in relationship to past time are we supposed to imagine something which brings it back to the memory or which excites its image and therefore so far causes us to contemplate it as present. Therefore, the emotion towards a contingent object which we know does not exist in the present, other things being equal, will be weaker than the emotion towards a past object.
In these propositions I consider that I have explained why men are more strongly influenced by an opinion than by true reason, and why the true knowledge of good and evil causes disturbance in the mind, and often gives way to every kind of lust, whence the saying of the poet, "Video meliora proboque, deteriora sequor." The same thought appears to have been in the mind of the Preacher when he said, "He that increaseth knowledge increaseth sorrow." I say these things not because I would be understood to conclude, therefore, that it is better to be ignorant than to be wise, or that the wise man in governing his passions is nothing better than the fool, but I say them because it is necessary for us to know both the strength and weakness of our nature, so that we may determine what reason can do and what it cannot do in governing our emotions.