The first aim of this paper is to make a thorough description of the use of nominals as well as of the use of the definite determiner in both Souletin (the most eastern dialect of Basque) and Standard Basque (plus the rest of Basque dialects). Then, assuming that Souletin is a previous stage compared to Standard Basque (cf. Michelena 1964, Camino to appear; cf. also Manterola 2012), the main aim of this paper is to explain how Basque historically moves from a situation where bare nouns (BNs) are allowed (in some restricted argument position and only in Souletin) to a situation where BNs are not allowed in argument position (Standard Basque). This paper argues that the reason we move from system A to system B is basically due to a semantic weakening and loss of the Souletin null D: (i) in Souletin BNs are full DPs with an empty head occupied by a phonetically null D--with indefinite reference and unspecified for number--, partly in line with Longobardi (1994, 2001); (ii) this null D loses its semantic features and since it is null it cannot be reanalyzed and gets lost; (iii) in Standard Basque, since null D is no longer available, the definite article [-a]--which will be argued to be a semantically flexible element--takes its place, as is phonologically a weak element, i.e. a suffix, and so the closest phonological alternative to the null D.