[0] is more immediately relevant, but the OP was probably referring in part to Kant's theory of noumena/phenomena [1]. By "absolute of all conditionals," they probably meant "the most final of all questions." Kant didn't mean it like this, but a good example is the following chain of questioning:

"Why is the sky blue?" -> "Why is the sky?" -> "Why is?"

Such questions, as Kant (and the OP) point out, ultimately hinge upon things not empirically observable (phenomena), but upon noumena instead.

[0]: https://plato.stanford.edu/entries/kant-spacetime/#AbsoVsRea...

[1]: https://plato.stanford.edu/entries/kant-transcendental-ideal...

I spent a minute looking at the results of that phrase on ddg, and got a hit on the book in question. While it's not a direct quote, a search of the book finds this line:

> On the one hand: We set limits to reason, preventing it from leaving the guiding-thread of empirical conditions and straying into the transcendent and explaining things in terms that can’t be cashed out empirically. And also, on the other hand (this being my present point): We set limits to the law of the purely empirical use of the understanding, preventing it from making decisions about the possibility of things in general—i.e. of things of any sort—and ruling that intelligible things are not merely incapable of explaining appearances but are impossible.

I assume that's what OP meant.