This paper completes a triad developed around the issue of scepticism in the enlightened thinking of Denis Diderot. The first one, entitled "The young Diderot and the scepticism of Thoughts ", was published in the journal Two Points , in its edition devoted to the subject of scepticism (cf. PIVA, 2007), and was limited to an analysis of the problem of attitude sceptical on Thoughts philosophical , in 1746. The second, "The clearing of Diderot with scepticism", published in the journal Transformation (cf. PIVA, 2009), deals with two fundamental questions, this time on The tour of the sceptic or The boulevards , of 1747, latest book in that the scepticism is raised to prominence by encyclopedist, since two years later he finally surrenders to the atheistic materialism. They are: 1) the interpretation that develops Diderot's scepticism and 2) its position before him. In this paper we will examine the presence of scepticism in thought diderotiano of maturity, namely the period beginning in 1749 with the drafting of the Charter for the blind , considering how they and where it is addressed records in some of the most significant texts its philosophical maturity.