Some great prose and statutory interpretation from Justice Kagan in Caraco Pharmaceutical v. Novo:

Truth be told, the answer to the general question “What does ‘not an’ mean?” is “It depends”: The meaning of thephrase turns on its context. See Johnson v. United States, 559 U. S. ___, ___ (2010) (slip op., at 5) (“Ultimately, context determines meaning”). “Not an” sometimes means “not any,” in the way Novo claims. If your spouse tells youhe is late because he “did not take a cab,” you will infer that he took no cab at all (but took the bus instead). If your child admits that she “did not read a book all summer,” you will surmise that she did not read any book (but went to the movies a lot). And if a sports-fan friend bemoans that “the New York Mets do not have a chance of winning the World Series,” you will gather that the team has no chance whatsoever (because they have no hitting). But now stop a moment. Suppose your spouse tells youthat he got lost because he “did not make a turn.” You would understand that he failed to make a particular turn,not that he drove from the outset in a straight line. Suppose your child explains her mediocre grade on a collegeexam by saying that she “did not read an assigned text.” You would infer that she failed to read a specific book, not that she read nothing at all on the syllabus. And supposea lawyer friend laments that in her last trial, she “did not prove an element of the offense.” You would grasp that she is speaking not of all the elements, but of a particular one. The examples could go on and on, but the point issimple enough: When it comes to the meaning of “not an,” context matters.6

6 For this reason, we find Novo’s reliance on the occasional dictionarydefinition of “a[n]” unconvincing. Although “an” sometimes means“any” when used in negative structures, see, e.g., Microsoft Encarta College Dictionary 1 (2001) (fifth definition), it sometimes does not. Cf. FCC v. AT&T Inc., 562 U. S. ___, ___ (2011) (slip. op., at 3–5) (rejecting a proposed definition of “personal” because it did not always hold in ordinary usage and the statutory context suggested it did not apply).