All the perceptions of the human mind resolve themselves into two distinct kinds, which I shall call IMPRESSIONS and IDEAS. The difference betwixt these consists in the degrees of force and liveliness, with which they strike upon the mind, and make their way into our thought or consciousness. Those perceptions, which enter with most force and violence, we may name impressions: and under this name I comprehend all our sensations, passions and emotions, as they make their first appearance in the soul. By ideas I mean the faint images of these in thinking and reasoning; such as, for instance, are all the perceptions excited by the present discourse, excepting only those which arise from the sight and touch, and excepting the immediate pleasure or uneasiness it may occasion. I believe it will not be very necessary to employ many words in explaining this distinction. Every one of himself will readily perceive the difference betwixt feeling and thinking. The common degrees of these are easily distinguished; tho' it is not impossible but in particular instances they may very nearly approach to each other. Thus in sleep, in a fever, in madness, or in any very violent emotions of soul, our ideas may approach to our impressions, As on the other hand it sometimes happens, that our impressions are so faint and low, that we cannot distinguish them from our ideas. But notwithstanding this near resemblance in a few instances, they are in general so very different, that no-one can make a scruple to rank them under distinct heads, and assign to each a peculiar name to mark the difference.
When Hume published the first volume of the Treatise in 1739, he couldn't have known that 260 years later, we would begin to learn that "impressions" (or sensations), and ideas, particularly those of the visual imagination, would turn out to share the same parts of the brain. Not only do they share some of the same brain regions, but there may be little difference in the brain activation caused by vivid impressions (sensations of external objects) and vivid visual images created by the imagination.
This has all sorts of implications for cognitive science. One of the most interesting implications concerns the formation of "false memories," which are now at the forefront of the repressed memory debate. For instance, in a
Researchers have known for some time that producing visual images through imagination can sometimes lead to memory errors, called "reality-monitoring errors" because they fail to distinguish between observed and imagined images. Some of this is likely to be due to retrieval problems, but some also may be due to how visual images, be they imagined or produced by the senses, are encoded. What this study found is that when false memories, or reality-monitoring errors, were produced for words that had been presented without pictures (i.e., when these words were remembered as having been presented with pictures), the brain areas that were active during the study phase (most notably, the precuneus, right parietal, and anterior cingulate regions) overlapped significantly with those active during actual picture presentation. These areas were significantly less active during the presentation of words during the study phase in cases that were subsequently remembered correctly as having been presented without pictures. False memories were produced for study words about four times as often than for words that had not been seen in the study phase.
The lesson, then, is that contrary to Hume, during both in encoding and retrieval, there is often little difference between visual images created by the senses and those created by the imagination, even the images created of objects when reading the words that refer to them (which Hume specifically claims is impossible or at least uncommon). The difference is small enough as to make it difficult to distinguish between sensory and imagined images in memory. What, exactly, this means for the repressed memory debate will require further research, but it clearly demonstrates that there is a neural basis for false memories produced by repeatedly imagining images, for instance through repeated suggestions or rumination. Since we've yet to come up with anything like a neural, or even an algorithmic or representational basis for memory repression of dissociation, this sort of advancement in memory research certainly doesn't bode well for the champions of repressed memory theories and recovered memory therapies.