Full Idea
Metaphysics only contains the pure a priori principles of physics in their universal import.
Gist of Idea
Metaphysics is just a priori universal principles of physics
Source
Immanuel Kant (Critique of Practical Reason [1788], I.II.II.VI)
Book Reference
Kant,Immanuel: 'Critique of Practical Reason (Third edition)', ed/tr. Beck,Lewis White [Library of Liberal Arts 1993], p.145
A Reaction
'Universal' seems to imply 'necessary'. If you thought that no a priori universal principles were possible, you would be left with physics. I quite like the definition, except that I think there would still be metaphysics even if there were no physics.