philosophers such as george berkeley and david hume, and early experimental psychologists such as wilhelm wundt and william james, understood ideas in general to be mental images. today it is very widely believed that much imagery functions as mental representations (or mental models), playing an important role in memory and thinking. william brant (2013, p. 12) traces the scientific use of the phrase "mental images" back to john tyndall's 1870 speech called the "scientific use of the imagination". some have gone so far as to suggest that images are best understood to be, by definition, a form of inner, mental or neural representation; in the case of hypnagogic and hypnapompic imagery, it is not representational at all. others reject the view that the image experience may be identical with (or directly caused by) any such representation in the mind or the brain, but do not take account of the non-representational forms of imagery.