Why were THE 25 years of philosophy precisely the ones between Kant and Hegel, per Dieter Henrich and older writings on them?
And what does this have to do with Kant and Hegel being first and foremost philosophers of modern freedom?
Philosophy Stack Exchange is a question and answer site for those interested in the study of the fundamental nature of knowledge, reality, and existence. It only takes a minute to sign up.Sign up to join this community
Short answer: Because of quotes by Kant and Hegel which indicate a span of 25 years between "a first" and "the conclusion of" philosophy. And this has absolutely nothing to do with freedom in particular.
This is made quite clear by the Henrich student (and his co-author) Eckart Förster in his The Twenty-Five Years of Philosophy (2012), Preface, page ix (bolded mine):
At the height of his career, in the preface to the Metaphysical Foundations of the Doctrine of Right, Kant declared that philosophy had not existed prior to the publication of his Critique of Pure Reason in 1781: “It sounds arrogant, self- seeking, and for those who have not yet relinquished their old system, belittling, to assert: that prior to the development of critical philosophy there had been no philosophy at all” (6:206)1. And yet he had no doubts about the truth of the statement he had committed to print and thus to posterity. As though that were not enough, Kant had concluded the Critique of Pure Reason, with which philosophy is sup- posed to have begun, by predicting that the completion of philosophy was now imminent and “it may be possible to achieve before the end of the century what many centuries have not been able to accomplish” (A856)— in less than nineteen years!
Things did not happen quite as Kant had predicted. Even so, it was a matter of fantastically few years before Hegel announced the end of the history of philosophy in a lecture held in Spring of 1806: “Herewith, this history of philosophy comes to an end” (TW 20:461).
Thus, the 25 years are an ironic commentary addressing these two quotes.
Well, all this is not about freedom, but about philosophy as a science. To give another, quite fitting quote of Förster's:
Since my concern here is to present not an historical survey, but the development of a thought, in what follows I will only deal with those receptions of Kant which agree with him in demanding that philosophy become a science, while objecting that this goal has not yet been achieved by Kant himself. From this point of view, the task of philosophy is to com- plete the project begun by Kant. “Philosophy is not yet at an end. Kant has given results; the premises are still missing. And who can understand results without premises?” Such was Schelling’s expression of this mood in a letter to Hegel on January 6, 1795.
As you can see in this answer of mine, the idea that philosophy should be a science is quite central to both Kant and Hegel. One of Hegel's endeavours has been to develop a philosophy that is based on a first principle - that what Kant was accused to be missing - and can thus be considered a science proper. Furthermore, the claim that there has not been philosophy before 1781 would be completely unintelligible if it was not for philosophy properly understood as scientific.
For more arguments about in which sense the philosophies of Kant and Hegel are scientific, see the linked answer and the first chapter of Förster's book.