David Francis Marks is a psychologist, author and editor of numerous articles and books largely concerned with five areas of psychological research – judgement, health psychology, consciousness, parapsychology and intelligence. Marks is also the originator of the General Theory of Behaviour, and has curated exhibitions and books about artists and their works.

In the philosophy of mind, neuroscience, and cognitive science, a mental image is an experience that, on most occasions, significantly resembles the experience of "perceiving" some object, event, or scene but occurs when the relevant object, event, or scene is not actually present to the senses. There are sometimes episodes, particularly on falling asleep and waking up, when the mental imagery may be dynamic, phantasmagoric, and involuntary in character, repeatedly presenting identifiable objects or actions, spilling over from waking events, or defying perception, presenting a kaleidoscopic field, in which no distinct object can be discerned. Mental imagery can sometimes produce the same effects as would be produced by the behavior or experience imagined.