On the intuitive notion of justification

Suppose we want to conceive a thought not, à la Frege, as something that can be true or false, but as something about which we are capable to establish what is a justification to believe it. Putting this characterization of the thought together with another one that can be found in Frege, according to which a thought is all that is necessary to know, or better to grasp, in order to understand a sentence, the idea emerges that understanding a sentence, grasping the thought it expresses, is tantamount to having a criterion for establishing what is a justification to believe it. From this we can extract the starting idea of a theory of meaning: the meaning of a sentence is a classification criterion which enables us to establish whether something is or not a justification to believe that sentence. Of course, this is only a very crude approximation; but I will not enter into an examination of how such a theory could be developed; professor Dummett has illustrated with the utmost clarity the general architecture of the theory. I will stop here and ask some questions concerning the intuitive notion of justification that is presupposed by the very program of a verificationist (or better justificationist) theory of meaning.