The subject matter of this article is a proposal of analysing the functioning of the exponents of epistemic modality which is based on the concept of the guarantor. The guarantor constitutes a foundation on which to take responsibility for a opinion concerning a given state of affairs. The guarantor appears in opinions which match the reality to a varying degree. In the article a distinction between the speaker-guarantor, the sender of the information, and the 'external' guarantor (with regard to the speaker) is made. It has been demonstrated that the problem of the guarantor is relevant in the context of lexical and grammatical means of expressing epistemic modality.