What is it? It’s the term applied by Einstein to the creative combination of (seemingly) unconnected theories to develop new, more general theories. Combinatorial play often works subconsciously, while a person is asleep or engaged in a “mindless” diversion. The possibility of arriving at a solution to a problem by shelving it — even overnight — underlies the counsel to “sleep on it.”
Combinatorial play occurs in the absence of artificial deadlines, which hamper truly creative thinking. Such thinking occurs when a person with deep knowledge of a practical, professional, or scientific subject acquires a new insight about the subject by thinking in new ways (often subconsciously) about his knowledge — by making new connections from “old facts,” so to speak.