What is really central in real thinking (its development)? I say: special and especially important PROXIMATE causes that are, at necessary times (points in development (ontogeny)), observable. ("Observable" both to the Subject and to the scientist. )
I submit that the real CORE (beginnings and THE BASES) of THINKING (itself) are certain (or a certain type of) PROXIMATE CAUSES and that, now with new eye-tracking technology, etc., these major directly observable proximate causes can be found with real-time study. THOSE THAT ARE ESPECIALLY IMPORTANT, during key points ("stages") in development (ages 1-18 y.o. +) (ontogeny): in rather "quick order" being obviously KEY in resulting (and realizing) new ways of categorizing and new ways to understand causation -- much of the point of THINKING. These would not only be proximate causes in the sense of something (here: environmental-aspects-and-associated-behavior-patterns) preceding something, that is, behavior[-pattern] change, BUT also in playing a distinct role in changing the nature of learning (actually: representation, memory, and learning). Thus, the great importance of likely then-OBSERVABLE (at that point in ontogeny) (via eye-tracking) perceptual/attentional shifts that usher in each new stage/level of representation (with memory changes) and new learnings, and soon shown through and/or with problem-solving