The only point remaining to make the analogy between covenants for title and warranty complete was to require assigns to be mentioned in order to enable them to sue. In modern times, of course, such a requirement, if it should exist, would be purely formal, and would be of no importance except as an ear-mark by which to trace the history of a doctrine. It would aid our studies if we could say that wherever assigns are to get the benefit of a covenant as privies in estate with the covenantee, they must be mentioned in the covenant. Whether such a requirement does exist or not would be hard to tell from the decisions alone. It is commonly supposed not to. But the popular opinion on this trifling point springs from a failure to understand one of the great antinomies of the law, which must now be explained. So far as we have gone, we have found that, wherever [381] one party steps into the rights or obligations of another, without in turn filling the situation of fact of which those rights or obligations are the legal consequences, the substitution is explained by a fictitious identification of the two individuals, which is derived from the analogy of the inheritance. This identification has been seen as it has been consciously worked out in the creation of the executor, whose entire status is governed by it. It has been seen still consciously applied in the narrower sphere of the heir. It has been found hidden at the root of the relation between buyer and seller in two cases at least, prescription and warranty, when the history of that relation is opened to a sufficient depth.