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Acting reflexively, not under conscious control. In English law the distinction between automatism and Insanity is drawn on the basis of internal and external factors, and is somewhat archaic. Automatism requires an external factor (e.g., a severe head wound). In an archetypal automatism case, Denning LJ classed sleepwalking as a meta of automatism; more recently it has had to be reclassified as 'insanity' because of the lack of an external factor.
Automatism may be a defence against a criminal charge, but not if the defendant was responsible for the situation that gave rise to the automatism (see: R v quick (1973)). In addition, there must be a total loss of conscious control, not merely an impairment (see: Ags ref 21992).
If the defendant introduces evidence of automatism, the prosecution have the task of disproving it beyond reasonable doubt. In insanity, however, the burden of proof is on the defendant.