In which publication does Bertrand Russell define "philosophy"?

Question

I've personally read a book or an article by Bertrand Russell where he defines philosophy. The definition --- IIRC --- is that philosophy is the study of the unknown. If it becomes a matter of fact, then it's called science.

Question. What is the publication where he mentioned that? (If you could, chapter, page number, edition would be great.)

I tried his autobiography. It could be there, but keywords such as philosophy, unknown, fact and obviously everywhere there. I've looked at his article on mathematical philosophy. It's likely not there. My best bet now is in The Scientific Outlook. It might very well be there, but I also can't find it.

Answer 1

Maybe Bertrand Russell, History of western philosophy, Introduction :

Philosophy, as I shall understand the word, is something intermediate between theology and science. Like theology, it consists of speculations on matters as to which definite knowledge has, so far, been unascertainable; but like science, it appeals to human reason rather than to authority, whether that of tradition or that of revelation. All definite knowledge—so I should contend—belongs to science; all dogma as to what surpasses definite knowledge belongs to theology. But between theology and science there is a No Man's Land, exposed to attack from both sides; this No Man's Land is philosophy. Almost all the questions of most interest to speculative minds are such as science cannot answer, and the confident answers of theologians no longer seem so convincing as they did in former centuries. [...]

To such questions no answer can be found in the laboratory. Theologies have professed to give answers, all too definite; but their very definiteness causes modern minds to view them with suspicion. The studying of these questions, if not the answering of them, is the business of philosophy.