“We may divide all the perceptions of the mind into two classes or species, which are distinguished by their different degrees of force and vivacity.” (Inquiry 1975, p18) These are the words of Hume, who maintains that all our perceptions can be categorized as either impressions or ideas and that there are a number of ways by which we can distinguish between them. He believes that not only do they differ in the way in which we feel them, but that impressions can be innate, while ideas cannot. He also states that we can distinguish between them mostly according to the level of force or vivacity that they display.
Such a theory has been disputed mostly because critics do not believe that the main difference between impressions and ideas can be found in their different degrees of vivacity. Other criticisms have focused on Hume's imagist theory of thinking. This essay will explain Hume's theory and examine its criticisms in order to discover how plausible a theory it is.
According to Hume, the mind is furnished with materials that consist of perceptions. In the Treatise, he determines a perception to be “whatever can be present in the mind, whether we employ our senses, or are actuated with passion, or exercise our thought and reflection,” (Treatise 1978, p647) which he subsequently divided into impressions and ideas. Impressions are established through sensation and emotion, while ideas are faint images of the impressions. An example of an impression would the sensation of heat and a burning pain if you put your hand in a flame. The idea is that the flame/fire is hot.
Hume implies that the distinction between the two corresponds to the distinction between feeling and thinking. In other words, impressions result both from direct sense experience (including pain, passions and emotions) and from remembered or imagined experience. By contrast, ideas in their first appearance are derived from impressions, which correspond to them and which they exactly represent. This is known as the copy principle, which is explained by Hume when he writes: “all perceptions of the mind are double, and appear both as impressions and ideas.” (Treatise 1978)
Hume also claims that there are no innate ideas. By comparison, he states that all impressions are innate. In Inquiry, he takes the term "innate" to mean what is original or not copied from a previous perception. However, this seems untrue in the case of impressions resulting from sense experience. Surely an impression of a fire is not innate? The impression that fire burns is a reaction to putting one's hand in a flame. Hume does not appear to respond to this criticism, other than to state that he is predominantly concerned with passions that are inherent in human nature, such as self-love.
Ideas, unlike impressions, represent objects which are not present within one's environment. Hume gives an example in the Treatise in order to explain this: he hears a person's voice from the room next door. His impressions (depending on his sound-sense) convey his thoughts to the person. I can paint a picture in my mind's eye of the person speaking and the surrounding objects. This is an idea. The sound of the man's voice is an impression, which gives rise to thoughts concerning the man and the objects around. It is clear that such thoughts are ideas because they do not relate to the subject's perceptual environment.
As stated above, the easiest way to distinguish between such impressions and ideas can be found in their differing degrees of “force and vivacity” (Treatise 1978) Hume introduces impressions as those perceptions which enter our consciousness with the most force, while ideas are faint images of such forceful impressions. He writes that impressions are “all our more lively perception[s], when we hear, or see, or feel, or love, or hate, or desire, or will.” (Inquiry 1975, p18) By contrast, ideas are fainter and weaker because they come into being “when we reflect on a passion or an object which is not present.” (Treatise 1978: p647) The distinction can be explained by using an example: when I see a tree I have an impression of it, but when I shut my eyes and I see an image of a tree in my mind's eye and therefore have an idea of it, the only difference between these two instances is the vivacity-degree.
If ideas are faint images of impressions, then this would imply that there is a certain similarity between what happens when I see the tree and when I have a picture of the tree, but that my picture of the tree is not as lively/clear as my impression of the tree, when I have a direct sense-experience of it. Hume tries to explain this by using a number of terms such as "vividness" and "forcefulness." However, such terms in this definition appear to be so compacted that it is easy to misunderstand. Rather than their force or vividness, it seems that an impression's most significant feature is its immediacy. Though this may have the effect of it being livelier, it is more likely that ideas are less lively because they are less immediate; they are not experienced first hand.
