Since Whorf (1936), many linguists have tried their hand at corralling the restrictions on the formation of "reversative" un-verbs; cf. e.g. Marchand (1969), Dowty (1979), Horn (1988), Clark et al. (1995). Why can you unwrap a sandwich but not unrecognize its contents or unremember to toss it in the trash? Why can a snake uncoil while a painting can't unhang? If unfreeze is the opposite of freeze, why is unthaw a synonym of thaw?
The standard approach to the constraints on un-verb formation invokes Whorf's CRYPTOTYPE -- a covert category encompassing transitive verbs of covering and enclosing that rules out a wide range of possible bases and outputs of the relevant rule. Pullum (1999), for example, reckons that there are "about a dozen verbs" that allow un-prefixation, citing undo (a good deed) and unknow as examples of formations we know "intuitively" are impossible. Clark et al. (1995) exclude unbury and unbend, while Kemmerer & Wright (2002) rule out unboil and undecorate. Yet many of the verbs depicted in the literature as impossible, non-occurring, or -- as in Whorf's label for unsay and unmake -- "semi-archaic" are readily attested, even when the actions they denote may be physically irreversible.
While the Pullum/Whorf view may be extended to predict the pleonastic interpretation of source-oriented reversatives in Swedish, French, and English (unloosen, unthaw), it incorrectly limits the productivity of un-verb formation by conflating the SEMANTIC (aspectual) restrictions with the PRAGMATIC conditions on the way the world (normally) works; verbs like unsay, unknow, unboil, and unhappen are motivated precisely by the need to describe those (typically counterfactual) situations in which the tape of reality is reversed.
The pragmatic nature of the restrictions on un-verb formation is supported by a survey of contexts that favor the emergence of innovative un-verbs: advances in science and technology (as in the unerase and undelete commands, the unfriend or unlike verbs of social networking, or the unfuck program to reverse software protection), science fiction (as in time-travel scenarios), advertising copy (as in KFC's current unthink campaign), and the imagination of poets from Shakespeare (whose "un-king’d" Richard II is the unchallenged monarch of this realm) to pop songsters ("How can I unlove you?", "Un-break my heart"), whether the implausibility of a given reversal is conceded, mourned, or overridden.
Larry Horn (Yale University)