The postulates, however, were taken by the economists to represent something more than approximate statements of the fact. They imply certain propositions which might be regarded as axioms. Men desire wealth and prefer their own interests. The whole theory might then be regarded as a direct deduction from the axioms. It thus seemed to have a kind of mathematical certainty. When facts failed to conform to the theory the difficulty could be met by speaking, as Malthus spoke, of 'tendencies,' or by appealing to the analogy of 'friction' in mechanics. The excuse might be perfectly valid in some cases, but it often sanctioned a serious error. It was assumed that the formula was still absolutely true of something, and that the check or friction was a really separable and accidental interference. Thus it became easy to discard, as irrelevant, objections which really applied to the principle itself, and to exaggerate the conformity between fact and theory. The economic categories are supposed to state the essential facts, and the qualifications necessary to make them accurate were apt to slip out of sight. Ricardo,[338] to mention a familiar instance, carefully points out that the 'economic rent,' which clearly represents an important economic category, is not to be confounded, as in 'popular' use, with the payments actually made, which often include much that is really profit. The distinction, however, was constantly forgotten, and the abstract formula summarily applied to the concrete fact.