David Hume

Enquiry Concerning Human Understanding

Section 2: The origin of ideas

Everyone will freely admit that the perceptions of the mind when a man Ÿfeels the pain of excessive heat or the pleasure of moderate warmth are considerably unlike what he feels when he later Ÿremembers this sensation or earlier Ÿlooks forward to it in his imagination. Memory and imagination may mimic or copy the perceptions of the senses, but they can’t create a perception that has as much force and vivacity as the one they are copying. Even when they operate with greatest vigour, the most we will say is that they represent their object so vividly that we could almost say we feel or see it. Except when the mind is out of order because of disease or madness, memory and imagination can never be so lively as to create perceptions that are indistinguishable from the ones we have in seeing or feeling. The most lively thought is still dimmer than the dullest sensation.   

A similar distinction runs through all the other perceptions of the mind. A real fit of anger is very different from merely thinking of that emotion. If you tell me that someone is in love, I understand your meaning and form a correct conception of the state he is in; but I would never mistake that conception for the turmoil of actually being in love! When we think back on our past sensations and feelings, our thought is a faithful mirror that copies
its objects truly; but it does so in colours that are fainter and more washed-out than those in which our original perceptions were clothed. To tell one from the other you don’t need careful thought or philosophical ability.

So we can divide the mind’s perceptions into two classes, on the basis of their different degrees of force and vivacity. The less forcible and lively are commonly called ‘thoughts’ or ‘ideas’. The others have no name in our language or in most others, presumably because we don’t need a general label for them except when we are doing
philosophy. Let us, then, take the liberty of calling them ‘impressions’, using that word in a slightly unusual sense. By the term ‘impression’, then, I mean all our more lively perceptions when we hear or see or feel or love or hate or desire or will. These are to be distinguished from ideas, which are the fainter perceptions of which we are conscious
when we reflect on our impressions.