Euclidian theories have it that there exist one or a small number of apex principles from which the entire fasciculus of rules of contract law can be logically deduced. Two arguments are marshalled against the Euclidian project. First, that it has been unsuccessfully attempted before – in the form of the nineteenth century contract law treatise which emulated the civil lawyer’s rationalistic model, mos geometricus – cautioning us against setting much store by its present reincarnation. Second, that the common law’s methodology makes it resistant to this form of theorising. Euclidian theory presupposes a picture of rules on which: a) cases involve an application of logically prior rules; b) rules are reliably identifiable by different actors in the legal system; and c) rules normatively range over an indefinite spectrum of future cases. It will be argued that the common law defies this picture of rules thus rendering Euclidian theory analytically impossible.