The concept of rationality, one might say, is incorrigibly elusive... I believe it is fair to say that in philosophical discussions of rationality, there is a sense in which we do not "know what we are talking about" and can never do so, if what is demanded is a concise definition (Max Black).
One of the slipperiest terms in the philosophical Lexicon, 'rationality' is many things to many people (Alvin Plantinga).
Assuming that Black and Plantinga are not too far off the mark on this, it would be foolish to assume that one can develop a theory of rationality where so many have failed without having some way of stacking the deck. Or, to carry the metaphor a step further, we might think of this in terms of discovering a 'tell' that Mother Nature has let slip that can give us some indication of the hand she might be holding -- some innocuous feature that others may have overlooked, perhaps,* that might serve to put the study of rationality on an empirical footing. And, in this regard, I have long been enamored of the premise that 'feelings of worthlessness'** are not so much an adaptation as a maladaptive byproduct of the evolution of rationality -- part of the price we humans have had to pay for having become a little too rational/ objective for our own good or -- in terms of the theory of natural selection -- part of the cost of doing business that Mother Nature "tolerates"*** as a necessary premium for having a rational species to do her bidding (e.g., perpetuating genetic blueprints).
The gist of the argument here is that, as humans became smarter and smarter about how the world is put together -- presumably transpiring over millennia of linguistic and cultural evolution -- they gradually became smarter and smarter about values. That is to say, contrary to Hume's famous dictum that reason is strictly a matter of truth and falsehood, they gradually became increasingly adept at distinguishing between values that are justifiable (e.g., supported by evidence) and those that are simply a matter of happenstance (e.g., products of natural selection). Eventually, so this story goes (and contrary to E. O. Wilson's famous dictum that values will inevitably be constrained in accordance with their effects on the human gene pool), this culminated in an increased volatility in the most crucial value of all -- one that in all likelihood lies at the very heart of the will to survive -- an increased volatility in self-value -- along with a host of maladaptive effects (anxiety, depression, addiction, suicide, guilt, etc.). In the form of an outline of a theory, this might read as follows:
General Observation: The species in which rationality is most developed is also the one in which individuals have the greatest difficulty in maintaining an "adequate" sense of self-worth, often going to extraordinary lengths in doing so (Evel Knievel, celibate monks, 9/11 terrorists, etc.).
General Hypothesis: Rationality is antagonistic to psychocentric stability (i.e., maintaining an "adequate" sense of self-worth).
Explanation: Being the blind arational process that she is (natural selection), Mother Nature instills in all her creatures a sense of their own importance (or of the importance of their needs) that is rationally inordinate (the maximum amount possible). And, as the members of a species reach a certain stage in their rational/ cultural/ linguistic development, they increasingly come to question this inordinacy (feelings of worthlessness), and increasingly come to require reasons (justification) for maintaining it manifested in a plethora of needs commonly conflated with "free will" (love, purpose, meaning, moral integrity, religion, achievement, status, etc.). In short, feelings of worthlessness are not so much an adaptation as a "maladaptive" byproduct of the evolution of rationality -- "maladaptive" in the sense of necessitating the expenditure of effort and energy, not on trying to stay alive, but on the biologically bizarre non-physical objective of maintaining self-worth.
The implications of this conjecture are extensive, including implications with respect to self-worth related emotional disorder (that it is, at least in part, a valuative affliction), indeterminism (that we are less valuatively/ conatively determined by natural selection than members of less rational more emotionally stable species), incompleteness (that, as with "seeing" that X is true, "seeing" that X is rationally inordinate is a matter of "standing outside the system" exemplified in Godelian arguments) and ethics:
Ethics: Since, according to this explanation, more rational correlates with more valuatively objective/ impartial, the moral maxim, 'love (intrinsically value) your neighbor as you love (intrinsically value) yourself' could be construed as an imperative of an implicit theory of rationality in which 'being rational' entails (among other things) being valuatively objective/ impartial. This would also mean that, to the extent this implicit theory turns out to be true,**** the author of Genesis might have actually gotten it right in referring to our awareness of right and wrong as a form of knowledge (moral realism), in this case, an emerging awareness of the nature of rationality itself.
*At one time the bible according to cognitive science, Gleitman's 'Psychology' serves up over 800 pages on everything from acquisition curves to zygotes, but with no references to such central features of human nature as feelings of worthlessness, guilt, self- esteem, etc. Similar oversights can be found in Eibl-Eibefeldt's 'Human Ethology' and Cosmides and Tooby's, 'The Adapted Mind'.
**For the metaphysically challenged, I am assumng that 'feelings of worthlessness' are an inter-subjectively reproducible non-physical feature of nature present exclusively in enculturated human minds. The wild boy of Aveyron and, prior to her famous 'Aha!', Helen Keller, may have experienced lots of feelings (pain, fear, aggression, frustration, despair, etc.), but I suspect that 'feelings of worthlessness' were probably not among them.
***In somewhat more literal terms, the maladaptiveness of an increased volatility in THE WILL to survive (feelings of worthlessness, depression, suicide, etc.) is outweighed by the adaptiveness of a massive increase in THE ABILITY to survive (agriculture, technology, global relief, etc.).
****It does. By assuming that, among other things, 'being rational' entails 'being objective', not only cognitively, but valuatively as well, it is possible to resolve a number of longstanding rationality paradoxes and perplexities (rational irrationality, epistemic vs. practical rationality conflict, the "rationality debate", the Prisoners' Dilemma, etc.), as I have demonstrated in 'Rationology 101'.