Where exactly Kant says ‘philosophy is to learn how to think …’?

Question

this is a famous quote from Emanuel Kant: “philosophy is to learn how to think, and not to learn thoughts”.

Where exactly did Kant say that? And what is the correct form of it?

Answer 1

In his "Nachricht von der Einrichtung seiner Vorlesungen in dem Winterhalbenjahre von 1765-1766" (Notification of the establishment of his lectures in the winter semester of 1765-1766) he writes the following (AA II 306):

Kurz, er soll nicht Gedanken, sondern denken lernen; man soll ihn nicht tragen, sondern leiten, wenn man will, daß er in Zukunft von sich selbst zu gehen geschickt sein soll.

In English (my own translation):

In short, he [the student of philosophy] should not learn thoughts but how to think; one should not carry, but lead him if one wants him to be able to walk by himself in the future.

Thus, he did write something very similar in the context of his pedagogical ideals of how to teach philosophy. This means he tells us how philosophy should be taught if we want to teach students to become philosophers in their own right, not about philosophy as such.

Answer 2

Nowhere. People are fond of putting cutesy platitudes into the mouth of somebody famous. He did write "to have trained one's children is not enough, rather, what really matters is that they learn to think" in Lectures on Pedagogy, see Zinkin, Kant on wonder as the motive to learn. See also collection of Kant's quotes on teaching style from notes of his lectures taken by others that express similar sentiments:

"One can thus learn philosophy, without being able to philosophize. Thus whoever properly wants to become a philosopher: he must make a free use of his reason, and not merely an imitative, so to speak, mechanical use. [...] How can one learn philosophy? One either derives philosophical cognitions from the first sources of their production, i.e., from the principles of reason; or one learns them from those who have philosophized. The easiest way is the latter. But that is not properly philosophy. Suppose there were a true philosophy, [if] one learned it, then one would still have only a historical cognition. A philosopher must be able to philosophize, and for that one must not learn philosophy; otherwise one can judge nothing. [...] One can make a distinction between the two expressions, to learn philosophy and to learn to philosophize. To learn is to imitate the judgments of others, hence is quite distinct from one’s own reflection."

Answer 3

Kant defines the term philosophy quite clearly in the introduction to Groundwork of the Metaphysics of Morals. He claims, that philosophy consists of Physics, Ethics and Logic. All of them are sciences according to him and divided into material and formal ones. So in short the answer would be no.