It is generally not recognized that Whewell's conception of necessary truth evolved only gradually; his early statements are misleading. For this reason, and because of certain peculiarities in his expository style over his publishing history, he is commonly thought to have used the term “necessary” in the sense of “absolutely necessary”. I argue that, on the contrary, the term is essentially relational in his mature view. This conclusion leads, in turn, to a re-interpretation of his doctrine of “fundamental ideas”. Here I argue that the crux of his position is that cognitive principles which are initially regulative become, through invention, repeated employment, and stipulative definition, constitutive ones.