Yes and No
The words “yes” and “no” are among the most familiar words of the English language, perpetually tripping off the tongue. But what do they mean—what kind of meaning do they have? They don’t have sense and reference: there is nothing they denote and there is no mode of presentation attached to them. They have no counterparts in established formal languages: no system of logic governs them. Theorists of language say nothing about them. They fall into no logical category: not singular terms, not predicates, not quantifiers, not connectives, not even brackets. No one talks about the logical form of yes-statements. Worse, they don’t appear to fall into any grammatical category: noun, verb, adjective, adverb, or preposition. Some linguists have classified them as sentences (“minor sentences”), because they get something linguistic done while standing alone; but even that must be wrong because they don’t compound as sentences do. You can’t negate them or conjoin them or insert them into a conditional. You can’t say “Not no” in response to the question “Would you like to go bowling?” or affirm “Yes and snow is white”. Some languages do without them in replies to questions (Finnish, Welsh), preferring instead to reiterate the verb of the question (“Are you coming?” “We are coming”). They seem a bit like “true” and “false” in expressing affirmation and negation, but those words behave like normal words, combining happily with other words as parts of real sentences (you can say “That’s true” but not “That’s yes”). The OED offers this for “yes”: “Used to give an affirmative response”; for “no” we have “Used to give a negative response”. The dictionary doesn’t specify what these words mean in the usual definitional style but instead indicates their use. We are assumed to understand what an “affirmative response” is—some sort of assent or consent behavior (likewise for “no”). We don’t normally employ these words in our inner speech, because their function is to indicate something to others not to act as vehicles of thought; presumably they would not exist in a purely individual language not used for communication. One might hazard that they are “expressive”, but what emotion do they express? They are not like a whoop of joy or a groan of disappointment. They appear anomalous, sui generis, and mildly suspect—oddballs, rule-breakers. Yet they are with us always, among the most natural of utterances. What is going on with these two little words?
I would call “yes” an assentive and “no” a dissentive. They are not alone in this neglected category: in addition to “yes” we have “yeah”, “yup”, “yep”, and “yah” (and for “no” we have “nope” and “nah”); but we also have “sure”, “right”, “ok”, “no problem”, and “definitely”. Moreover, we can dispense with the vocal organs altogether in registering our assent or dissent: we can nod or shake our head, smile or frown, or point our thumb up or down. There are lots of ways to show you feel favorably or unfavorably towards something. Couldn’t we just dispense with “yes” and “no” and get by with body language? These points all nudge us in the direction of the following conjecture: “yes” and “no” are not words at all (nor phrases or sentences). They simply don’t function like words: they have no grammar, no combinatorial power; they are not part of the computational system that other words participate in. They have a communicative use, to be sure, but that is not sufficient to make them part of language proper, defined as a certain formal structure—what Chomsky would call the human language faculty. Animal communication systems have their uses too, but they are not languages in this restricted sense—infinite recursive generative rule-governed grammatical systems. Strictly speaking, “yes” and “no” have no semantics and no syntax—they are not words in the proper sense. They obviously have their uses, but they are not semantic-syntactic particles (and hence neither nouns nor verbs nor adjectives nor adverbs nor prepositions). They signify but they don’t mean (except in the sense of speaker- meaning). Put differently, they have no conceptual interpretation and no representational function.
This suggestion may appear radical and counterintuitive, but actually there is considerable precedent for it: for speech is full of such “meaningless” elements. Consider “oh”, “ah”, “ooh”, “ha”, “hey”, “um”, “uh”, and “er”: these all occur frequently in speech but they are not words. Sometimes they occur in writing too, but only as a way to mimic speech: they look like words but they aren’t words. Indeed, they are not really elements of speech construed as the vocalization of words: they are speech helpers or auxiliaries or props. They are ersatz words. And they combine naturally with “yes” and “no” in informal speech: “Oh yes”, “Uh, no”. They can both also be repeated for emphasis: “yeah yeah yeah”, “Ha ha”. This is like nodding vigorously or emphatically wagging one’s finger. We can modulate our response so as to indicate strong assent or firm dissent: the response can vary in magnitude (words proper don’t do that). Speaking loudly can also communicate state of mind, but nobody thinks that volume is a word. Linguists sometimes call these devices “paralinguistic”: “yes” and “no” evidently share several features with the paralinguistic. They are quasi words, borderline words, words by courtesy only.
Here is a hypothesis: assent and dissent are important behaviors in a social species such as ourselves, predating the arrival of the human language faculty; the particles “yes” and “no” are just the latest way to get such attitudes across to conspecifics. We used to nod and wag, smile and grimace, but now we say “yes” and “no”: this is considered polite, civilized, well bred. We are communicating our attitudes of assent and dissent, consent or rejection, using the latest piece of human technology, viz. vocal speech. But we are harking back to more primitive times when we used other means to convey our attitudes. In animal mating behavior, assent and dissent clearly play an important role; the human “yes” and “no” are devices for getting these preferences across (among other devices). Presumably other species have their own methods for conveying assent and dissent, which are not verbalized; well, we are playing much the same game. Saying “yes” and “no” is just one way to indicate affirmative and negative response, but such responses are part of our pre-linguistic history; and the words (sic) carry this history within them. They represent the survival of an ancient signaling system within our newfangled capacity for articulate speech—along with assorted paralinguistic devices. What we loosely call “speech” is really an amalgam of evolutionary adaptations not a unified trait, and “yes” and “no” straddle these disparate systems. This is why we tolerate so much variation of pronunciation in these (putative) words: because we just need to convey assent or dissent not home in on a specific lexical item. If you mispronounce “house” you risk misunderstanding, but you can indicate assent in many verbal (and non-verbal) ways and not be criticized for it. This is also why the Beatles used “yeah” so often in their songs: it represents a more primordial state of mind than regular words. The “yeah” sound is joyful and optimistic, indicating harmony, consent, and agreement (no Beatles song has “Nah nah nah” in the chorus); it indicates a positive state of mind, extra-linguistically. Cavemen are often depicted as communicating by means of grunts: this has psychological truth to it in that non-linguistic communication goes to our more basic instincts. The grunt is universal and easily understood. “Yes” is the most beautiful word in the English language precisely because it isn’t really a word—it isn’t a component of that formal computational system that came into existence a mere 200,000 years ago. Cooperation is the sine qua non of a social species, so expressions of affirmation are of the essence. Our word “yes” packs all of that into its short span (“no” is its unwelcome sidekick). It is a profoundly loaded word without really being a word at all (a combinatorial grammatical unit). We could do without it so long as we were adept at non-verbal communication (perhaps the Welsh and the Finns are). Say no to “yes”, but do so without saying “no”. “Yes” and “no” correspond to primitive acts, biologically based; the words are just recent tokens or tags.
 This shows that “yes” and “no” are not inter-definable using negation, unlike “true” and “false”: “yes” can’t mean “not no” and “no” can’t mean “not yes”—simply because these are not well formed. This is why we never use such locutions, though we can of course say, “I’m not saying yes” and “I’m not saying no”. These latter two sentences are curious in their own right, since they are using “yes” and “no” when they should be mentioning them. Any logically aware writer is uncomfortable with such sentences. Language is trying to squeeze “yes” and “no” into ordinary sentence frames. It’s like saying, “He said hello”, which is ambiguous at best.
 In the Geordie dialect we have “why aye” in which “why” does not have its usual meaning. Presumably it is the rhyme that makes this form attractive to speakers (“Are you going to see Sunderland play today?” “Why aye, man”).
 Shakespeare has King Lear utter the following “sentence” at the death of Cordelia: “O, O, O, O!” This is “language” reduced to the level of the grunt—but in context a sublime grunt. Compare “Yes!” uttered in jubilation.
 More accurately, that is when human speech entered human history, but the language faculty could have predated vocal speech by a long time, perhaps used for the purpose of enhancing thought.
 Of course the same story could be told for “si” and oui” and the rest: all these phonetic units are surrogates for the act of affirmation.