environmental statute, for example, defines a floating home (for regulatory purposes) as “a single-family dwelling unit constructed on a float, that is moored, anchored, or otherwise secured in waters, and is not a vessel, even though it may be capable of being towed.” Wash. Rev. Code Ann. §90.58.270(5)(b)(ii) (Supp. 2012). A California statute defines a floating home (for tax purposes) as “a floating structure” that is “designed and built to be used, or is modified to be used, as a stationary waterborne residential dwelling,” and which (unlike a typical houseboat), has no independent power generation, and is dependent on shore utilities. Cal. Health & Safety Code Ann. §18075.55(d) (West 2006). These States, we are told, treat structures that meet their “floating home” definitions like ordinary land-based homes rather than like vessels. Brief for Seattle Floating Homes Association 2. Consistency of interpretation of related state and federal laws is a virtue in that it helps to create simplicity making the law easier to understand and to follow for lawyers and for nonlawyers alike. And that consideration here supports our conclusion.
D
The City and supporting amici make several important arguments that warrant our response. First, they argue against use of any purpose-based test lest we introduce into “vessel” determinations a subjective element—namely, the owner’s intent. That element, they say, is often “unverifiable” and too easily manipulated. Its introduction would “foment unpredictability and invite gamesmanship.” Brief for Respondent 33.
We agree with the City about the need to eliminate the consideration of evidence of subjective intent. But we cannot agree that the need requires abandonment of all criteria based on “purpose.” Cf. Stewart, 543 U.S., at 495 (discussing transportation purpose). Indeed, it is difficult,
