  It is often argued that reasoning by analogy and distinguishing precedents are mirror images of each other: given the facts of two cases, the question is whether there is a good reason for treating them differently (e.g. Eisenberg 1988, 87). In the case of distinguishing, a precedent must be followed unless there are good reasons for treating it differently. In the case of analogy, it is said, a precedent must be extended unless there are good reasons for treating the instant case differently. But this is misleading, since the symmetry is incomplete. A precedent cannot be distinguished on grounds that would, in substance, imply that the precedent was wrongly decided: it must be treated as correctly decided. A precedent need not be extended, however, if later courts regard its rationale as unpersuasive. A longstanding doctrine of the common law was that a husband could not commit the offence of rape against his wife. By the twentieth century the rule was increasingly recognised to be archaic and objectionable.  Thus, unlike the question of impersonation discussed above, no one suggested that the rule should be extended to cohabiting  couples.[18]   Where a decision is not regarded as misguided, however, it does provide an argument for being followed. The later court may still decide, however, that it would not be desirable, all things considered, to do so.