A principle propounded by Gareth Evans in his influential book The Varieties of Reference (1982) according to which ‘if a subject can be credited with the thought that a is F, then he must have the conceptual resources for entertaining the thought that a is G, for every property of being G of which he has a conception’. The principle needs some qualification to accommodate different categories of things about which we can think: for example it makes sense to call a geometric figure a pentagon, but not an interval of time.