Arthur Koestler in 1964 coined the term “bisociation”- the simultaneous perception of two hitherto unrelated frames of reference or , more simply, the discovery of hidden similarities- to capture the great “aha” moment.
In the 3rd century BC, Archimedes was asked by King Ieron of Syracuse to find whether his newly-made crown was real gold or whether the craftsman had mixed in any “base” metals. Immersed in his bathtub, Archimedes saw the water level rise and, realizing he had his solution, jumped out and ran around the town naked shouting “Eureka! Eureka!” (“I have found it”- the solution) Archimedes understood that by immersing a real gold object equal in weight to the crown, and comparing the water level to the level resulting from an immersion of the crown, he could verify whether they had the same density, ie if they were of the same metal.
The creative moment was when two completely unrelated things (level of bathwater, material of crown) came together in the creator’s mind.