Full Idea
It is a clear contradiction to try to extract necessity from an empirical proposition.
Gist of Idea
Necessity cannot be extracted from an empirical proposition
Source
Immanuel Kant (Critique of Practical Reason [1788], Pref)
Book Reference
Kant,Immanuel: 'Critique of Practical Reason (Third edition)', ed/tr. Beck,Lewis White [Library of Liberal Arts 1993], p.12
A Reaction
This is precisely the idea which Kripke challenged, claiming that the necessary essences of natural kinds such as gold have to be discovered empirically. All my intuitions are with Kant (and Hume) on this, but it is a complex issue…