“These last weeks at Turin, where I shall stay till June 5th, have turned out better than any I have known for years, above all more philosophic. Almost every day for one or two hours I have packed such a pitch of energy as to be able to view my whole conception from top to bottom; so that the immense multiplicity of problems lies spread out beneath me, as though in relief and clear in its outlines. This requires a maximum of strength, for which I had almost given up hope, It all hangs together; years ago it was already on the right course, one builds one’s philosophy like a beaver, one is forced to and does not know it. But one has to see all this, as I have now seen it, in order to believe it.”
That’s from a letter to Brandes, 4 May 1888, quoted over here.