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In the third and final volume of his groundbreaking 'Handbuch der physiologischen Optik' (1856-76, translated as 'Treatise on Physiological Optics' in 1920-25), the great physicist and polymath Hermann von Helmholtz discussed the psychological effects of visual perception. According to Helmholtz, an unconscious conclusion ('unbewusster Schluss') is an involuntary, pre-rational and reflex-like mechanism which is part of the formation of visual impressions. Obstinately and unswervingly, this mechanism follows its own rule and thus wields an imperious mastery over the human mind. Every evening we see the sun go down behind the motionless horizon although we are well aware that it is the earth that rotates around the fixed sun, rather than the other way around. What is more, we are unable to do away with such optical illusions simply by convincing ourselves rationally that our eyes have played tricks on us.

Likewise, people's perceptions of each other are deeply influenced by unconscious conclusions. The mere sight of another person is sufficient to produce an emotional attitude without any reasonable basis whatsoever, yet highly resilient against all rational criticism. Obviously, the impression is based on the spontaneous attribution of traits - a process we can hardly avoid, for the human eye, so to speak, is incapable of doubt and thus cannot ward off the impression created by the unconscious conclusion.

The reason, Helmholtz suggested, lies in the way visual sensory impressions are processed neurologically. The higher cortical centres responsible for conscious deliberation are not involved in the formation of visual impressions. However, as the process is spontaneous and automatic, we are unable to account for just how we arrived at our judgments. Through our eyes, we necessarily perceive things as real, for the results of the unconscious conclusions are interpretations which "are urged on our consciousness, so to speak, as if an external power had constrained us, over which our will has no control".

In recognizing these attitude-formation mechanisms underlying the human processing of nonverbal cues, Helmholtz anticipated developments in science by more than a century. More recent authors have since approached Helmholtz's conception under a variety of headings, such as 'snap judgments' (Schneider, Hastorff and Ellsworth, 1979), 'nonconscious social information processing' (Lewicki 1986), 'spontaneous trait inference' (Newman and Uleman 1989), 'people as flexible interpreters' (Newman, Moscowitz and Uleman 1989), and 'unintended thought' (Uleman and Bargh 1989).