In Aristoteles (Aristotle) it is a yearning that we share with animals. Therefore, it is different from the purposive choice (<prohairesis>) which is a human function that animals do not manage. In fact, <prohairesis> involves both reason and deliberation.
Moreover, <orexis>/<ekklisis> are the subjects of the first of the three <topoi>, areas of training (<askesis>), that produce self-coherence, the other two being <horme>/<aphorme> and <synkatathesis>.
The Stoics indicate <pathos> and <eupathos> as respectively, an irrational (<alogon>) example and a reasonable (<eulogon>) example of the four kinds of psychic motion (<kineseis>).
• Firstly, ‘reaching’ (<orexis>) in response to a prospective apparent good.
• Secondly, ‘shrinking away’ (<ekklisis>) in response to a prospective apparent evil.
• Thirdly, ‘swelling’ (<eparsis>) in response to a present apparent good.
• Finally, ‘contraction’ (<sustole>) in response to a present apparent evil.
<Orexis> is ‘desire’ but the notion of desire is not easy to define and requires some hands-on experiences.
According to Margaret Graver (2007), the four Stoic motions are kinds of “affective response”. Other scholars believe that only psychic swellings and contractions are affective responses. Those professors think that psychic swellings and contractions appear as a result of the success or failure of <orexis> and <ekklisis>. Hence they say that <orexis> and <ekklisis> are more purely behavioral impulses of pursuit and avoidance.
Anyway this topic is very complex for all academics, being very difficult to investigate with the necessary objectivity.