 In doing so we may aim to capture much in earlier views while keeping in mind that pleasure is something biological, psychological, and experiential which remains in large part unknown, the nature or category of which it is inappropriate to stipulate a priori. Perhaps pleasure expresses the unimpeded functioning (Aristotle) of our Natural anxiety-free and pain-free State (Epicurus) by which we are able to reach outward from our hedonic core to engage with more representational brain processes – and through these, with love, to all the world (citations in n. 25). But perhaps pleasure has a more complex reflexive intentional structure, as suggested in some of the medieval literature mentioned in §2.3.1 and n. 20  ad loc., and understanding the self-organization of recurrent neural activity will someday help us to introspect this better. Elements at least of these suggestions and others are compatible. Or perhaps pleasure divides in two, perhaps along the lines between the ‘wanting’ and ‘liking’ discussed in the preceding section, no one natural kind responding to all we intuitively seek, with dopaminergic reward needed to organize our exploratory pleasures of pursuit until we are ready for opioid bliss and repose. But we should also not forget more humble and basic biological facts: that mood varies with energy and thus with circadian rhythms affecting body temperature and also with the current availability of nutrients in the blood (Thayer 1989, 1996); that how much pleasure we experience also depends on getting enough, and good enough, sleep; that pleasure increases immune response (Rosenkranz et al, 2003), and that how we feel may grow in part out of monitoring bodily homeostasis (Craig 2002, 2009, 2015). These facts are telling about what may, perhaps, turn out to be more a syndrome of typically causally connected features than a simple or unified psychobiological phenomenon, such as would better fit philosophers’ penchant for simple kinds and simple explanations.