Although we intuitively understand the meaning of such terms as ‘reason’ and ‘rationality’, these concepts are not unequivocal (e.g. this SEP article on bounded rationality lists at least 7 accounts of rationality and says that the list is not exhaustive). Moreover, these concepts often have a normative connotation where being rational and reasonable is considered good, valuable, worth striving for.
If I try to remain as value-free as possible, I would look at these concepts from a biological perspective. Here, ‘reason’ can be regarded as a cognitive faculty that an organism may have at its disposal. Such faculty, then, can make rational thought possible. This sort of thought usually requires more cognitive effort than, for instance, instinct-based decision making. Therefore, one of the defining qualities of rational thought could be its more resources-demanding and, thus, slower nature than, say, emotion or intuition-based decision. For instance, if I see a wolf approaching me in a forest, my decision for further action will be immediate and based on an instant sense of fear for my life, with no laborious deliberation involved.
This recognition of reason-based thought as being something effortful also appears in philosophy professor Peter Adamson’s interpretation of al-Kindī’s arguments about the knowledge gained by prophets through divine revelation as opposed to knowledge gained by any other human through reasoning (i.e. for al-Kindī, through study and practice of philosophy). In his 2007 book “Al-Kindī”, Adamson writes (my emphasis):
“The human sciences (al‐‘ulūm al‐insāniyya), he says, “are of a lower rank [martaba] than divine knowledge [al‐‘ilm al‐ilāhĪ],” because the latter can be acquired “without study, effort, or human methods, and without time” (§VI.1, AR 372) The prophets simply “know through the will of Him, the great and exalted, and by their souls’ being purified and illuminated with the truth” (§VI.2, AR 373). Indeed, their instant and effortless access to the truth is a proof that their knowledge does indeed come from God (§VI.3, AR 373)… al‐KindĪ is here favorably contrasting prophetic knowledge to “human” or philosophical knowledge. But he does not say that prophets have access to any more or different knowledge from that attained in philosophy. Rather prophets have access to precisely the same truths, but instantly and without effort or study… Revelation sets out its truths in a more precise, brief, and compelling way than any human could hope to achieve. But revelation does not add truths which are unavailable to human reason (as would be held by, say, Thomas Aquinas). Nor does it fall short of philosophy by failing to provide rigorous demonstration, and by being merely rhetorical or symbolic, intended for the masses who cannot hope to master philosophy (as al‐FārābĪ and Averroes would have it).”P. Adamson (2007), pp43-44
For many of us today, this approach to defining rationality may seem slightly counterintuitive. After all, the Enlightenment movement has taught the Western world that the effortful rational argumentation of one’s thoughts is the noble way towards gaining knowledge. However, if we take a step back and look at it from the intellectual distance afforded by the historical perspective, we will realise that such normative evaluation has not always been the case and is not a necessary given.