 Weatherson  (2013;  see  Other Internet Resources)  has a related objection to the claim that naturalness, understood as playing certain other roles we have already discussed (e.g. in the analysis of duplication) also makes for projectibility: he argues that being green is highly projectible whereas being an electron is not highly projectible, since merely finding that all observed electrons have a certain mass does not strongly support the conclusion that all electrons have that mass, unless one acquires further evidence which supports the hypothesis that being an electron is very natural. Opponents of externalism about mental content, such as Lewis  (1981),  might respond that their theory of induction is in the first instance about rationality in the formation of narrow beliefs and credences, and propositions like all electrons have mass \(m\) are not narrowly believed (see the entry on  narrow mental content).  But externalists about content will also need to be cautious about embracing Weatherson’s argument, since if they want to speak of “degrees of projectibility” at all, there is pressure for them to say that a single property can be denoted by different concepts with different degrees of projectibility.