Unconscious inference

Unconscious inference (German: unbewusster Schluss), also referred to as unconscious conclusion, is a term of perceptual psychology coined in 1867 by the German physicist and polymath Hermann von Helmholtz to describe an involuntary, pre-rational and reflex-like mechanism which is part of the formation of visual impressions. While precursory notions have been identified in the writings of Thomas Hobbes, Robert Hooke, and Francis North (especially in connection with auditory perception) as well as in Francis Bacon's Novum Organum, Helmholtz's theory was long ignored or even dismissed by philosophy and psychology. It has since received new attention from modern research, and the work of recent scholars has approached Helmholtz's view.

In the third and final volume of his Handbuch der physiologischen Optik (1856–67, translated as Treatise on Physiological Optics in 1920-25), Helmholtz discussed the psychological effects of visual perception. His first example is that of the illusion of the sun rotating around the earth:

Every evening apparently before our eyes the sun goes down behind the stationary horizon, although we are well aware that the sun is fixed and the horizon moves.

We are unable to do away with such optical illusions by convincing ourselves rationally that our eyes have played tricks on us: obstinately and unswervingly, the mechanism follows its own rule and thus wields an imperious mastery over the human mind. While optical illusions are the most obvious instances of unconscious inference, people's perceptions of each other are similarly influenced by such unintended, unconscious conclusions. Helmholtz's second example refers to theatrical performance, arguing that the strong emotional effect of a play results mainly from the viewers' inability to doubt the visual impressions generated by unconscious inference:

An actor who cleverly portrays an old man is for us an old man there on the stage, so long as we let the immediate impression sway us, and do not forcibly recall that the programme states that the person moving about there is the young actor with whom we are acquainted. We consider him as being angry or in pain according as he shows us one or the other mode of countenance and demeanour. He arouses fright or sympathy in us [...]; and the deep-seated conviction that all this is only show and play does not hinder our emotions at all, provided the actor does not cease to play his part. On the contrary, a fictitious tale of this sort, which we seem to enter into ourselves, grips and tortures us more than a similar true story would do when we read it in a dry documentary report.

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