Perelman & Olbrechts-Tyteca discuss the effects of argumentation on the basis of an opposition between to persuade and to convince, the former being a local achievement involving a particular audience, while the latter is a global achievement involving the universal audience. The functional definition of argumentation provided at the opening of the Treatise, however, does not use these concepts but speaks of “adherence of minds” and “assent”. In this passage, argumentation is seen as an activity aiming “to induce or to increase the mind’s adherence” to “theses” that are “presented for its assent” (Perelman & Olbrechts-Tyteca, 1958, p. 4).

The concept of assent refers to Newman’s Grammar of Assent (1870).

The Stoic theory of knowledge defines assent as a voluntary act of the soul which occurs when the soul receives a true impression; this process implies a pre-established harmony between the will and the mind. “The soul wants truth”, and truth is index sui, its own mark. The mark of the true impression is the assent granted to it. The skeptics reject this harmony between true representation and assent; truth is not capable of self-certification, that is, one can give its assent to false representations.

The suspension or abstention of assent, is the basis of the skeptical method to achieve tranquility (ataraxia):

The Skeptic Way is called […] aporetic either, as some say, from its being puzzled and questioning about everything or from its being at a loss as to whether to assent or dissent. (Sextus Empiricus, Outlines, I, iii)

Assent may be granted or refused by an act of will:

I think it a very great exploit to resist one’s perceptions, to withstand one’s vague opinions, to check one’s propensity to give assent to propositions; […] Carneades achieved a Herculean labor when, as it had been a savage and formidable monster, he extracted assent, that is to say, vague opinion and rashness from our minds. (Cicero, Ac. II, 34; Trans. Yonge, p. 74)

Skepticism characterizes the argumentative situation as a standoff between two equal (isosthenic) and opposed discursive forces, which imposes a suspension of assent, S. Force; Stasis.

Common language considers assent to be an action. Assent can be given or suspended, in the same way that one can give or suspend an agreement or an authorization. From a rhetorical point of view, the problematic of assent makes the concept of persuasion more complex, by granting some activity to the recipient. Whilst people are passively persuaded, they actively grant their assent. This maintains a balance between the speaker and the audience, in that the speaker’s effort to persuade his or her audience corresponds the audience’s capacity to grant or to refuse his or her assent. Withheld assent plays a role in all varieties of rational exchanges, as it brings about a state of doubt which characterizes the third party position, S. Roles.

The assent granted in regard to a proposition is characterized by varying degrees, as one moves from opinion to belief and knowledge:

— The lowest degree corresponds to opinion, defined as a belief accompanied by the awareness that there are other equally valid opinions.
— The intermediate degree is that of belief. There are other beliefs, considered not false, but less valid than one’s own belief.
— The strongest degree is that of conviction. The convinced party considers that the proposition to which he or she adheres is true and that opposing arguments are fallacious, perverse or insane.

According to Perelman & Olbrechts-Tyteca, persuading produces belief, while convincing produces a generalized belief, defining social, legitimized knowledge.