  All of the theories considered so far have treated desire as a suitable topic for a priori investigation, with one exception.  Morillo's methodology treats desires as a natural kind: the natural kind that is responsible for a familiar effect—action—one associates with desire. This general methodology is shared by another philosopher, Timothy Schroeder. But Schroeder sees the neuroscientific evidence as supporting a different conclusion from Morillo's. Schroeder agrees with Morillo that there is a single neurological structure that is the unique common cause of the various phenomena associated with desire (at least, with action, pleasure, and some aspects of thought and attention), and agrees that this structure is the dopamine-releasing reward system. However, he argues that the activity of this neurological structure does not realize pleasure.  (Pleasure is said to be one of its effects.) Rather, the output of this neurological structure realizes a form of unconscious learning known as contingency-based learning or reward-based learning. Schroeder thus concludes that desires are this natural kind: reward-based learning mechanisms. Desires have all of their familiar effects on action, feelings, and thought only contingently (compare Morillo 1990 to Schroeder 2004; compare both to the related view in Butler 1992).