Principle needed in the attempt to develop a logic of probability, in the Treatise on Probability (1921, ch. 22) by Keynes. It assures us that ‘the objects in the field, over which our generalisations extend, do not have an infinite number of independent qualities; that, in other words, their characteristics, however numerous, cohere together in groups of invariable connection, which are finite in number’. The principle is needed if inductive methods, such as Mill’s methods, are to give us trustworthy conclusions; more strictly, we need a finite probability that an object about which we seek to generalize is not infinitely complex in the way it excludes. Keynes was well aware of the apparently metaphysical and unverifiable nature of the principle.