 Yet propensities seem to be measures of ‘causal tendencies’, and much as the causal relation is asymmetric, so these propensities supposedly do not reverse. Suppose that we have a test for an illness that occasionally gives false positives and false negatives. A given sick patient may have a (non-trivial) propensity to give a positive test result, but it apparently makes no sense to say that a given positive test result has a (non-trivial) propensity to have come from a sick patient. Thus, we have an argument that whatever they are, propensities must not obey the usual probability calculus. ‘Humphreys’ paradox’, as it is known, is really an argument against any formal account of propensities that has as a theorem: