Evidentiality is a set of grammatical or linguistic phenomena indicating how the information conveyed in a statement has been obtained by the speaker. Evidential systems commonly signal that the information 1) comes from sensory experience (auditory, visual); 2) that is has been inferred from something else; 3) that it reproduces a hearsay. Other evidential systems are much more complex. In evidential languages, the speaker must explicitly mention the basis on which he or she says what he or she says, that is, the kind of argument supporting the utterance.

In some languages, evidentiality is grammaticalized, that is, it corresponds to a specific grammatical category. In English, for example, the reported event is necessarily referred to by its temporal-aspectual coordinates. In evidential languages, the speaker is obliged to stipulate how the information he relays has been obtained (via the senses, hearsay, inference, etc.). The sub-system of the grammatical marks of evidentiality is distinct from the modal system as well as from the temporal-aspectual system.

Evidentiality can be considered as a linguistically embedded argumentation, as an “argumentation within grammar”. It leads to the conception of the argumentation as a continuum, sometimes related to the grammar and semantics of the language and at other times, to the grammar and semantics of discourse.

In English, where evidentiality is not grammaticalized, evidential markers or phrases remain optional. The evidential sources can be discursively expressed as coordinated sentences or as the head of the sentence:

Peter is at home;

one can hear him             I hear that Peter is at home
one can see him               I see that —
they told me                   They told me that —
I read it                          I read that —
I guess                          I guess that —

Evidentiality may be expressed by modals. So, for example, in the statement “Peter is at home”, the information about that Peter’s whereabouts is given in the categorical mode, and is endorsed by the highest degree of speaker certainty on an epistemic scale ranging from doubt to certitude. From an evidential perspective, the statement implies that the speaker has some direct evidence to back the speech, for example “I just left him”, etc.

Peter should be at home now”: this sentence communicates the same information on a lower position on the epistemic scale. From an evidential perspective, the statement implies, for example, “I have no direct and categorical evidence of what I say, but on the basis of Peter’s usual habits, I infer that he is at home”.

The following example is taken from Ducrot (1975). In “Pierre doit avoir reçu ma lettre” (“Peter must/should have received my letter”), , the information is backed only by common knowledge of the usual delivery deadlines. The following case is different:

Eh bien, je crois que Pierre a reçu ma lettre!
Well, I believe that Peter has received my letter!

There is now an implication that the same information has been inferred from a quite different source, that is, some natural sign taken in Peter’s behavior which can be explained only by referring to the letter’s content; for example, the letter informed Peter of a disciplinary warning, and Peter clearly changed his behavior.