Origin and history of unreason

c. 1200, resoun, "the intellectual faculty that adopts actions to ends," also "statement in an argument, statement of explanation or justification," from Anglo-French resoun, Old French raison "course; matter; subject; language, speech; thought, opinion," from Latin rationem (nominative ratio) "reckoning, understanding, motive, cause," from ratus, past participle of reri "to reckon, think" (from PIE root *re- "to reason, count").

The meaning "sanity; degree of intelligence that distinguishes men from brutes" is recorded from late 13c.; that of "that which recommends itself to enlightened intelligence, a reasonable view of a matter" is from c. 1300.

The sense of "grounds for action, motive, cause of an event" is from c. 1300. The Middle English sense of "meaning, signification" (early 14c.) is preserved in the phrase rhyme or reason. For stands to reason see stand (v.). A reason of state (1610s) is a purely political grounds for action.

The Enlightenment gave reason its focused sense of "intelligence considered as having universal validity ... so that it is not something that belongs to any person, but is something partaken of, a sort of light in which every mind must perceive" [Century Dictionary]. Reason itself has long been personified, typically as a woman. Age of Reason "the European Enlightenment" is by 1794 as the title of Tom Paine's book.

"They that seek a reason of all things, do utterly overthrow reason." [Hooker, translating Theophrastus ('Metaphysics'), in "Ecclesiastical Polity," 1594]

Reason is never a root, neither of act nor desire.

[Robinson Jeffers, "Meditation on Saviors"]

prefix of negation, Old English un-, from Proto-Germanic *un- (source also of Old Saxon, Old Frisian, Old High German, German un-, Gothic un-, Dutch on-), from PIE *n- (source of Sanskrit a-, an- "not," Greek a-, an-, Old Irish an-, Latin in-), combining form of PIE root *ne- "not."

The most prolific of English prefixes, freely and widely used in Old English, where it forms more than 1,000 compounds. It disputes with Latin-derived cognate in- (1) the right to form the negation of certain words (indigestable/undigestable, etc.), and though both might be deployed in cooperation to indicate shades of meaning (unfamous/infamous), typically they are not.

Often euphemistic (untruth for "a lie") or emphatic, if there is a sense already of divestment or releasing: unpeel " to peel;" unpick "pick (a lock) with burglars' tools;" unloose for "to loosen."

It also makes words from phrases, such as uncalled-for, c. 1600; undreamed-of, 1630s. Fuller (1661) has unbooklearned. A mid-15c. description of a legal will has unawaydoable; Ben Jonson has un-in-one-breath-utterable. The word uncome-at-able is attested by 1690s in Congreve, frowned at by Samuel Johnson in the 18th century and by Fowler in the 20th ("The word had doubtless, two or three centuries ago, a jolly daredevil hang-the-grammarians air about it ; that has long evaporated ; it serves no purpose that inaccessible does not ....").

But the practice continued; unlawlearned (Bentham, 1810), unlayholdable (1860); unputdownable, of a book, is by 1947; unpindownable, by 1966. Also compare put-up-able-with (1812). As a prefix in telegraphese, to replace not and save the cost of a word, it is attested by 1936.

With the variety of its possible use, and the need for negatives, the number of un- words that might be made in English is almost endless, and that some are used and some never is owing to the caprice of authors.

Dictionary editors noted this since 18c. but also padded the list. John Ash's "New and Complete Dictionary of the English Language" (1775) has many pages of one-line un- entries; among a dozen consecutive entries are unhaggled, unhaired, unhalooed, unhaltering (adj.), unhaltering (n.), which sorts of words OED (1989) notes were "obviously manufactured for the purpose" and some turn up in other texts only decades later, if at all. (Ash vindicated.)