I. A. Richards begins his Principles of Literary Criticism with the claim that "a book is a machine to think with." He does not seem to want it to usurp the role of a locomotive or of the bellows, however. His book is meant to be a loom by means of which he tends to "reweave some ravelled parts of our civilisation." Whether "re-weaving" already qualifies as thinking should be questionable—or so it seems to me. Furthermore, I am worried with Kant that many people will substitute a book—and this does not have to be the Holy Bible—for thinking: Once you have a book do the thinking for you, you do not have to think for yourself any longer! Still, I think the claim is very suggestive. I would slightly vary it and say: "a notebook is a machine to think with." Such a notebook does not have to be made of paper, of course. But it must be the kind of thing that allows you to confront what you have noted and thought in a form that is independent of your own subjective frame of mind at any given moment. No matter how primitive this machine is, thinking with it is better than bare thinking. To quote Francis Bacon one more time: "Neither the naked hand nor the understanding left to itself can effect much. It is by instruments and helps that the work is done, which are as much wanted for the understanding as for the hand. And as the instruments of the hand either give motion or guide it, so the instruments of the mind supply either suggestions for the understanding or caution." Richards' "speculative instruments" are his "eyes": the theories, principles, methods, codes, and poems by means of which the mind extends its power of understanding are not just indebted to Bacon's "instruments of the mind," but also presuppose externalization.
The phrase "speculative instruments" seems to come from Samuel Taylor Coleridge, who kept many “memorandum books” for the development of his own speculative instruments.
1. There is a fine study of the role some of them played in his work by John Livingston Lowes, The Road to Xanadu. A Study in the Ways of the Imagination (Boston and New York Houghton Mifflin and The Riverside Press Cambridge, 1927).