In this essay I will discuss the issue of vagueness when defining the concept of open work within the philosophy of Umberto Eco (1932-2016), particularly considering its relevance for the development of his original semiotic view. The analysis of vagueness allows us to stress the importance of Eco’s concept of open work not only in Opera Aperta (1962) but in different phases of his thought. This paper is divided into five sections. Section 1 briefly outlines the Peircean notion of vagueness, (...) trying to understand it as a pivotal concept to define the structure and dynamical form of the open work. Section 2 dwells on the concept of quality in both John Dewey and Charles Sanders Peirce: here the possibility of articulating an interpretation is considered embodied in the vague immediacy of the qualitative experience. Section 3 analyses to what extent an artwork can be considered open and in motion, according to the Pareysonian concepts of form and interpretation. Section 4 stresses the hypothesis that vagueness produces not only the condition of possibility of an open work and of its multiple interpretations, but also an increase of information due to its aesthetic quality. Section 5 calls into question the distinction between use and interpretation and specifies that interpreting an open work does not mean using it for your own purposes but interpreting a developmental movement towards its own fulfilment; a text is nothing other than the rule that constitutes the universe of its interpretations. (shrink)