Max Wertheimer

Psychologist (1880–1943), born in Prague into a German-speaking Jewish family, whose short 1912 paper on the perception of movement founded Gestalt psychology. The story of its origin is well attested: travelling by train in 1910 he got off at Frankfurt, bought a toy stroboscope, and began to test what happens when two lights are flashed in succession at the right interval — the observer sees not two lights but one light moving, and at a shorter interval a pure movement with nothing moving in it, which he named the phi phenomenon.

The Experimentelle Studien über das Sehen von Bewegung reported the result with Wolfgang Köhler and Kurt Koffka as his subjects, and drew from it the conclusion that made a school: the experience cannot be assembled out of the two sensations, because there is nothing in either of them that corresponds to motion, so the organised whole must be the primary thing and the elements a later abstraction.

His 1923 essay on the laws of grouping set out the principles by which a field organises itself — proximity, similarity, closed form, good continuation, common fate, all subordinate to the tendency towards the simplest and most stable structure he called Prägnanz. He taught at Berlin from 1916 and at Frankfurt from 1929, and left Germany within weeks of the Nazi seizure of power in 1933 for the New School for Social Research in New York, where he worked until his death.

Productive Thinking, published posthumously in 1945, applied the same idea to reasoning: genuine understanding is a restructuring of the whole problem, not a chain of associations, and its case studies include long conversations with Albert Einstein about how the theory of relativity had actually been thought.