Students of philosophy tend to associate transcendentalism with Immanuel Kant, who argued against metaphysical skepticism on grounds that reason necessarily presumes both time and substance. Kant had no monopoly on "transcendental," however, which simply describes a particular form of argument. A transcendental argument begins with an uncontroversial fact, adds a proposition that necessarily follows from that fact, and concludes in support of the proposition. Following that form, the transcendental argument for consent's moral relevance runs as follows:
1. A justification aims to win the consent of its intended audience.
2. If a justification aims to win the consent of its intended audience, then the argument's efficacy covaries with the consent of that audience.
3. Therefore, justification presumes the value of consent.
This argument for consent's moral relevance begins with a (supposed) truism about the nature of justification. Readers who regard step one as an obvious truth can skip to step two without delay. Some might doubt its truth, however; in particular, a skeptic might counter that justifications sometimes aim to mislead their intended audiences, as when political leaders conspire to mislead gullible citizens about the causes of social unrest, blaming foreign provocateurs rather than native disaffection. In such a case, however, we cannot properly say that the justification aims to win the consent of the governed; it aims, rather, to win their ignorant acquiescence.
The claim made in step two of the transcendental argument for consent's role in justification might, like the claim made in step one, strike many readers as obvious. As long ago as Aristotle, philosophers have regarded the end, or teleos, of a thing as a fair gauge of its proper function. On that reasoning, if a justification (or, more properly, the person offering the justification) aims to win the consent of a particular audience, we can judge whether or not the argument succeeds by measuring the consent that the argument rouses.
The third step of the argument for the moral relevance of consent follows as a matter of logic from the first two steps. Even hardcore skeptics do not trouble themselves challenging modus ponens, so perhaps we could stop here. As a safeguard against sophistry, however, let us double-check whether the argument's conclusion—that justification presumes the value of consent—conforms with common sense.
Note, first, that an argument nobody accepts cannot work as a justification. We thus laugh off the arguments, no matter how internally consistent or ardently pressed, a madman makes when he claims the right to rule the Earth. Because his argument wins nobody's consent, nobody regards it as sufficient justification for his coronation. Note, next, that we commonly regard informed consent as adequate justification for imposing far-ranging conditions on those who accept them; we hesitate to second-guess another's pursuit of happiness. Lastly, note that we tend to recognize exceptions to that rule only in defense of consent itself, as when we refuse to enforce an agreement to submit to slavery, when we deny the power of fraud to justify a transaction, or when, far from praising a mugger for successfully inducing his victim to give up her purse in exchange for not losing her life, we condemn his acts as coercive and unjustified. Logic and experience alike thus suggest that we judge an attempted justification in terms of whether or not it wins the consent of its intended audience. Unsurprisingly, the plain meaning of "justify" conforms to that understanding.
[NB: The foregoing comes, after various edits, from Part I.B. of Graduated Consent Theory, Explained and Applied, Chapman University School of Law, Legal Studies Research Paper Series, Paper No. 09-13 (March 2009) [PDF].]