Full Idea
The Idealist saw in Kant that knowledge, which depends on the spontaneity of judgement, and self-determined spontaneous action, can be seen as sharing the same source, which is not accessible to scientific investigation.
Gist of Idea
The Idealists saw the same unexplained spontaneity in Kant's judgements and choices
Source
Andrew Bowie (German Philosophy: a very short introduction [2010])
Book Reference
Bowie,Andrew: 'German Philosophy: very short intro' [OUP 2010], p.33
A Reaction
This is the 'spontaneity' of judgements and choices which was seen as the main idea in Kant. It inspired romantic individualism. The judgements are the rule-based application of concepts.