  Aristotle investigates psychological phenomena primarily in De Anima and a loosely related collection of short works called the Parva Naturalia, whose most noteworthy pieces are De Sensu and De Memoria. He also touches upon psychological topics, often only incidentally, in his ethical, political, and metaphysical treatises, as well as in his scientific writings, especially De Motu Animalium. The works in the Parva Naturalia are, in comparison with De Anima, empirically oriented, investigating, as Aristotle says, “the phenomena common to soul and body” (De Sensu 1, 436a6–8). This contrasts with De Anima, which introduces as a question for consideration “whether all affections are common to what has the soul or whether there is some affection peculiar to the soul itself” (De Anima i 1, 402a3–5). That is, in De Anima Aristotle wants to know whether all psychological states are also material states of the body. “This,” he remarks, “it is necessary to grasp, but not easy” (De Anima i 1, 402a5). In this way, De Anima proceeds at a higher level of abstraction than the Parva Naturalia. It is generally more theoretical, more self-conscious about method, and more alert to general philosophical questions about perception, thinking, and soul-body relations.