Full Idea
Kant stresses that reason, when it turns dialectical, posits immutable basic entities; these are the standard inhabitants of traditional a priori metaphysics - God, souls, Platonic ideas, Democritean indestructible atoms, and the like.
Gist of Idea
A priori metaphysics is fond of basic unchanging entities like God, the soul, Forms, atoms…
Source
report of Immanuel Kant (Critique of Pure Reason [1781]) by Robert Fogelin - Walking the Tightrope of Reason Ch.3
Book Reference
Fogelin,Robert: 'Walking the Tightrope of Reason' [OUP 2004], p.89
A Reaction
This sounds like a good warning, but it just invites the meta-question in a priori metaphysics 'Are we searching for something unchanging, or is this impossible?' Aristotle certainly addressed this question. The search strikes me as sensible.