Alison Gopnik, in the most gonzo—and a touch overinclusive—chapter titled "Explanation as Orgasm and the Drive for Causal Knowledge: The Function, Evolution, and Phenomenology of the Theory Formation System," argues that explanation, or uncovering causes, is to theory formation as orgasm is to reproduction; or perhaps as uncovering the spatial character of moving objects is central to the visual system. She argues that explanation is a drive (is this a return of the repressed old libido theory of Freud?); that is, our genes provide orgasm, and explanation, because reproduction, and theory (a veridical causal map of the world) are good for us in an evolutionary sense. Just as there is a phenomenology of emotions, Gopnik postulates a distinctive phenomenology of explanation, which she reduces to the "hmm" and "aha" experiences corresponding to "why," the search for explanation, and "because," the recognition that explanation has been reached. These are close, but not identical, to curiosity and interest, and Gopnik postulates distinctive facial expressions for each. She places the drive for causal knowledge in a child developmental context. For example, the terrible twos are a child’s experiments with differences in desires, even though they may elicit a mother’s rage; this is a drive going beyond mere cognition. Gopnik extrapolates that "in Swann’s Way, Swann compulsively tests Odette in search of her secret life, in spite of the emotional and practical pain this will cause him, a rather advanced case of ‘the terrible twos’ " (p. 312).