Full Idea
For Kant, the conceptual apparatus that structures our experience for us will inevitably lead to intellectual disasters when it is applied to matters completely beyond experience.
Gist of Idea
For Kant, our conceptual scheme is disastrous when it reaches beyond experience
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.71
A Reaction
This is the empiricist side of Kant, influenced by Hume. I don't agree with Kant on this. I just think that speculation and abstract theory are much more difficult and error-prone than science, because you can't keep checking against raw facts.