Reason involves the ability to think, understand and draw conclusions in an abstract way, as in human thinking. The meaning of the word "reason" overlaps to a large extent with "rationality" and the adjective of reason in philosophical contexts is normally "rational", not "reasonable".
The concept of reason is closely related to the concepts of language and logic, as reflected in the multiple meanings of the Greek word "logos", the root of logic, which translated into Latin became "ratio" and then in French "raison", from which the English word "reason" was derived. In contrast to reason more generally, language refers not to the thinking as such, but to the communication or potential communication of rational thoughts.
Reason and logic can be thought of as distinct, although logic is one important aspect of reason. Reason is a type of thought. Logic, as the word is used in modern languages, is the attempt to make explicit the rules by which reason operates. The oldest surviving writing to explicitly and at length consider the rules by which reason operates are the works of the Greek philosopher Aristotle, especially Prior Analysis and Posterior Analysis. Although the Ancient Greeks had no separate word for logic as distinct from language and reason, Aristotle's neologism "syllogism" (syllogismos) identified logic clearly for the first time as a distinct field of study. When Aristotle referred to "the logical" (hê logikê), he was referring more broadly to rational thought.
Author Douglas Hofstadter, in Gödel, Escher, Bach, characterizes the distinction in this way. Logic is what is done "inside the system" by formal steps such as deduction. Reason is what is done "outside the system" by such informal methods as skipping steps, working backward, drawing diagrams, looking at examples, or seeing what happens if you change the rules of the system.
Another way to consider the confusion between logic and reason is that computers and animals sometimes perform actions which are apparently logical: from a complex set of data, conclusions are achieved which are "logical". Being a cause of something which humans find logical does not necessarily mean that computers or animals have reason, or even logic in the strict sense. Some animals are also clearly capable of a type of "associative thinking" - even to the extent of associating causes and effects. A dog once kicked, can learn how to recognize the warning signs and avoid being kicked in the future. Human reason is something much more specific, requiring not just the possibility of associating perceptions of smoke, for example, with memories of fire, but also the ability to create and manipulate a system of symbols, or icons in the terminology of Charles Peirce, which have only a nominal connection to either smoke or fire.
Thomas Hobbes described the creation of “Markes, or Notes of remembrance” (Leviathan Ch.4) as “speech” (allowing by his definition that it is not necessarily a means of communication or speech in the normal sense; he was clearly using "speech" as an English version of "logos" in this description). In the context of a language, these marks or notes are called "Signes" by Hobbes.
Since classical times a question has remained constant in philosophical debate (which is sometimes described or taught as a conflict between movements called Platonism and Aristotelianism) concerning the role of reason in confirming truth.
Both Aristotle and Plato, like many philosophers throughout history, did indeed write about this question, which can be explained as follows. On the one hand, people use logic, deduction, and induction, to reach conclusions they think are true. Conclusions reached in this way are considered more certain than sense perceptions on their own. On the other hand, if such reasoned conclusions are only built originally upon a foundation of sense perceptions, then, the argument being considered goes, our most logical conclusions can never be said to be certain because they are built upon the very same fallible perceptions they seek to better.
This leads to the question of what types of first principles, or starting points of reasoning, are available for someone seeking to come to true conclusions. Empiricism (sometimes associated with Aristotle but more correctly associated with British philosophers such as John Locke and David Hume, as well as their ancient equivalents such as Democritus) asserts that sensory impressions are the only available starting points for reasoning and attempting to attain truth. This approach always leads to the controversial conclusion that absolute knowledge is not attainable. Idealism, (associated with Plato and his school), claims that there is a "higher" reality, from which certain people can directly arrive at truth without needing to rely only upon the senses, and that this higher reality is therefore the primary source of truth.
In Greek, “first principles” are arkhai, starting points, and the faculty used to perceive them is sometimes referred to in Aristotle and Plato as “nous” which was close in meaning to “awareness” or “consciousness”.
Among those who would argue that reason can not be based upon experience alone, at least two major strands might be discerned. On the one hand, philosophers such as Plato, Aristotle, Alfarabi, Avicenna, Averroes, Maimonides, Aquinas and Hegel are sometimes said to have argued that reason must be fixed and discoverable - perhaps by dialectic, analysis, or study. In the vision of these thinkers, reason is divine or at least has divine attributes. Such an approach allowed religious philosophers such as Thomas Aquinas and Étienne Gilson to try to show that reason and revelation are compatible.
On the other hand, since the Seventeenth Century rationalists, reason has often been taken to be a subjective faculty, or rather the unaided ability (pure reason) to form concepts. For Descartes, Spinoza and Leibniz, this was associated with mathematics. Kant attempted to show that pure reason could form concepts (time and space) that are the conditions of experience. Kant made his argument in opposition to Hume, who denied that reason had any role to play in experience.
Reason is then more than just what people can communicate. It is also necessary to discuss what it means to the human imagination. For imagination is not only found in humans. Aristotle, for example, stated that phantasia (imagination: that which can hold images or phantasmata) and phronein (a type of thinking which can judge and understand in some sense) also exist in some animals. Both are related to the primary perceptive ability of animals, which gathers the perceptions of different senses and defines the order of the things that are perceived without distinguishing universals, and without deliberation or logos. This is equivalent to the habitual thinking about cause and effect discussed by Hume, and mentioned above. But this is not yet reason, because human imagination is different.
The recent modern writings of Terrence Deacon and Merlin Donald fit into an older tradition which makes reason connected to language, and mimesis, but more specifically the ability to create language as part of an internal modeling of reality specific to humankind. Other results are consciousness, and imagination or fantasy. In more recent times, important areas of research include the relationship between reason and language, especially in discussions of origin of language. Modern proponents of a priori reasoning, at least with regards to language, include Noam Chomsky and Steven Pinker, to whom Donald and Deacon can be usefully contrasted.
If reason is symbolic thinking, and peculiarly human, then this implies that humans have a special ability to maintain a clear consciousness of the distinctness of "icons" or images and the real things they represent. Starting with a modern author, Merlin Donald writes…
A dog might perceive the “meaning” of a fight that was realistically play-acted by humans, but it could not reconstruct the message or distinguish the representation from its referent (a real fight). [...] Trained apes are able to make this distinction; young children make this distinction early – hence, their effortless distinction between play-acting an event and the event itself
What Donald refers to here can be compared to Plato's term, eikasia. Jacob Klein’s A Commentary on the Meno is a commentary for a particularly difficult Plato dialog concerning learning, the Meno which contains a long digression on this difficult subject. According to this, an important aspect of human thinking in the Ancient Greek philosophical terminology of Plato is eikasia. This is the ability to perceive whether a perception is an image of something else, related somehow but not the same, and which therefore allows us to perceive that a dream or memory or a reflection in a mirror is not reality as such. What Klein refers to as dianoetic eikasia is the eikasia concerned specifically with thinking and mental images, such as those mental symbols, icons, "signes" and marks which are discussed above as definitive of reason. Explaining reason from this direction: human thinking is special in the way that we often understand visible things as if they were themselves images of our intelligible "objects of thought" as "foundations" (hypothêses in Ancient Greek). This thinking (dianoia) is "an activity which consists in making the vast and diffuse jungle of the visible world depend on a plurality of of more 'precise' noêta.
In turn, both Merlin Donald and the Socratic authors emphasize the importance of mimesis, often translated as “imitation”. Donald writes
Imitation is found especially in monkeys and apes [… but…] Mimesis is fundamentally different from imitation and mimicry in that it involves the invention of intentional representations. [...] Mimesis is not absolutely tied to external communication.
Mimêsis is a concept, now popular again in academic discussion, which was particularly prevalent in Plato’s works, and within Aristotle, it is discussed mainly in the Poetics. In Michael Davis’s account of the theory of man in this work.
It is the distinctive feature of human action, that whenever we choose what we do, we imagine an action for ourselves as though we were inspecting it from the outside. Intentions are nothing more than imagined actions, internalizings of the external. All action is therefore imitation of action; it is poetic......Thus Davis is here using “poetic” in an unusual sense, questioning the contrast in Aristotle between action (praxis, the praktikê) and making (poêsis, the poêtikê)...
...Human [peculiarly human] action is imitation of action because thinking is always rethinking. Aristotle can define human beings as at once rational animals, political animals, and imitative animals because in the end the three are the same.
We can also note that Donald also shares with Plato and Aristotle (especially in On Memory and Recollection), an emphasis upon the peculiarity in humans of voluntary initiation of a search through one’s mental world. The ancient Greek anamnêsis, normally translated as “recollection” was opposed to mneme or “memory”. Memory, shared with some animals, requires a consciousness not only of what happened in the past, but also that something happened in the past, which is in other words a kind of eikasia "but nothing except man is able to recollect. Recollection is a deliberate effort to search for and recapture something which was once known. Klein writes that, to "become aware of our having forgotten something means to begin recollecting.
Donald calls the same thing “autocueing”, which he explains as follows.
Mimetic acts are reproducible on the basis of internal, self-generated cues. This permits voluntary recall of mimetic representations, without the aid of external cues – probably the earliest form of representational “thinking”.
In a celebrated paper on this subject of modern times, the fantasy author and philologist J.R.R. Tolkien wrote in his essay "On Fairy Stories" that the terms “fantasy” and “enchantment” are connected to not only “the satisfaction of certain primordial human desires” but also “the origin of language and of the mind”.
It is also common, particularly since Freud, to describe reason as the servant of the passions - the means of sorting out our desires and then getting what we want, or perhaps even the slave of the passions - allowing us to pretend to reason to the object of our desire. Such feigned reason is called "rationalization".
Philosophers such as Plato, Rousseau, Hume, and Nietzsche have combined both views - making rational thinking not only a tool of desires, but also something privileged within the spectrum of desires, being itself desired, and not only because of its usefulness in satisfying other desires.
Modern psychology has much to say on the role of emotions in belief formation. Deeper philosophical questions about the relation between belief and reality are studied in the field of epistemology, which forms part of the philosophical basis of science, a branch of human activity that specifically aims to determine (certain types of) truth by methods that avoid dependence on the emotions of the researchers.
Though no theology or religion claims to be irrational, there is often a perceived conflict or tension between faith and tradition on the one hand, and reason on the other, as potentially competing sources of wisdom, law and truth. Defenders of traditions and faiths from claims that they are irrationalist for ignoring or even attempting to forbid reason and argument concerning some subjects, typically maintain that there is no real conflict with reason, because reason itself is not enough to explain such things as the origins of the universe, or right and wrong, and so reason can and should be complemented by other sources of knowledge. The counter claim to this is that such a defense does not logically explain why arguments from reason would be forbidden or ignored.
There are enormously wide differences between different faiths, or even schools within different faiths, concerning this matter.
Some commentators have claimed that Western civilization can be almost defined by its serious testing of the limits of tension between “unaided” reason and faith in "revealed" truths - figuratively summarized as Athens and Jerusalem, respectively. Leo Strauss spoke of a "Greater West" which included all areas under the influence of the tension between Greek rationalism and Abrahamic revelation, including the Muslim lands. He was particularly influenced by the great Muslim philosopher Al-Farabi. In order to consider to what extent Eastern philosophy might have partaken of these important tensions, it is perhaps best to consider whether dharma or tao may be equivalent to Nature (by which we mean physis in Greek). According to Strauss the beginning of philosophy involved the "discovery or invention of nature" and the "pre-philosophical equivalent of nature" was supplied by "such notions as 'custom' or 'ways'" which appear to be "really universal" "in all times and places". The philosophical concept of nature or natures as a way of understanding arkhai (first principles of knowledge) brought about a peculiar tension between reasoning on the one hand, and tradition or faith on the other.