Unconscious phenomena are those mental phenomena which their possessor cannot introspect, not only at the moment at which the phenomenon occurs, but even when prompted (‘Do you think/want/…?’). There are abundant allusions to many kinds of unconscious phenomena from classical times to Freud. Most notably, Plato in his Meno defended a doctrine of anamnesis according to which a priori knowledge of, for example, geometry is ‘recollected’ from a previous life. But the notion of a rich, unconscious mental life really takes hold in nineteenth-century writers, such as Herder, Hegel, Helmholtz and Schopenhauer. It is partly out of this latter tradition that Freud’s famous postulations of unconscious, ‘repressed’ desires and memories emerged.
Partly in reaction to the excesses of introspection and partly because of the rise of computational models of mental processes, twentieth-century psychology has often been tempted by Lashley’s view that ‘no activity of mind is ever conscious’ (1956). A wide range of recent experiments do suggest that people can be unaware of a multitude of sensory cognitive factors (for example, pupillary dilation, cognitive dissonance, subliminal cues to problem-solving) that demonstrably affect their behaviour. And Weiskrantz has documented cases of ‘blindsight’ in which patients with damage to their visual cortex can be shown to be sensitive to visual material they sincerely claim they cannot see.
The most controversial cases of unconscious phenomena are those which the agent could not possibly introspect, even in principle. Chomsky ascribes unconscious knowledge of quite abstract principles of grammar to adults and even newborn children that only a linguist could infer.
Many philosophers have found these claims about the unconscious unconvincing, even incoherent. However, they need to show how the evidence cited above could be otherwise explained, and why appeals to the unconscious have seemed so perfectly intelligible throughout history.
Pupillary dilation elicited by attending to two disks with different luminance
