In Worlds Apart, Sanderson the anthroposopher wonders "if it may not be possible for thinking to be a part of nature without its having been produced by her? Couldn't it be somehow correlative to nature?" (WA 42). Barfield's works answer the question in the affirmative.1

"It is . . . a conclusion which is commonly forgotten by . . . philosophers, psychologists, neurologists, and physicists almost as soon as it has been arrived at; or certainly as soon as they turn their minds to other matters--such as history or evolution--but which I personally decline to forget. I mean the conclusion, the irrefragable consensus, that what we perceive is structurally inseparable from what we think" (HGH 14). Barfield does not forget. He retains a profound faith (affirmed in "Science and Quality") that

Only in thinking--in pure thinking as distinct from an abstract chain of thoughts based on remembered sense-impressions--the individual human being functions, not as a skin-confined personality, but as anthropos. That is really the heart of the matter: that the less personal a man is--the less merely and egotistically personal--the more truly individual he is. And the more individual he is, the more universal he is. (RM 180)
The nature of thinking has evolved. "Without going back to the age of Myth-Consciousness," Barfield writes in "Man, Thought and Nature," "a sensitive reading of, for example, Greek philosophy can make us feel very vividly how thinking was in former days, not merely theoretically supposed to be, but actually experienced as a kind of nature process going in a man's environment, in which his own mind participated" (RCA 229). But we need not go back to the ancients to note the evolution in progress. "read a lecture by T. H. Huxley . . . or a speech by Gladstone," Sanderson observes in Worlds Apart, "and then read something by a contemporary exponent of science or a contemporary politician. The thoughts are altogether thinner. . . . The trouble today is that we have all got very clever. . . . We are no longer capable of thinking deeply, because we think too quickly" (57).2

"When we think about the word Thinking today," Barfield writes, "we ordinarily mean by it something which is confined within our skins or, if you like, in a corner of our brains."3  Though "it is not true to say that the human mind, and particularly its less conscious processes, are cut off from the processes of nature because the former go on inside our skins and the latter outside them," it is now "true to say that the thinking of which we are fully conscious is now focused or centered in the brain in a way which does cut us off from nature and enables us to feel ourselves, at any rate, as definitely not a part of nature" (WA 138). Such a stance is inherently paradoxical, for, as Barfield detects,

what is so very singular about the epistemological outlook of our generation is: that on the one hand they have the idea that the whole configuration, colour, appearances, and cohesion of nature depends on Thinking and yet, on the other, they have the idea that this Thinking is no more than a kind of a flicker going on in a corner of the brain. (RCA 227)
Barfield asks, however, that we suspend our interiority, surmount our reductionism, come out of our room of one's own, and "imagine [thinking] coming to mean something very different."
Just as we look back to a time before Kepler and Newton, when Gravity had such a cramped and parochial meaning--quite other than the spacious one we now attach to it--so, I am persuaded that our descendents will look back, perhaps with amusement, to a time when Thinking and Thought had the strangely cramped and parochial meaning it has today. Because, for them Thinking will be something as to which one simply takes it for granted that it permeates the whole world of nature and indeed the whole universe. (RCA 226)
All of Barfield's discoveries about the nature of thinking come together in the following passage, a self-referential speech (Barfield cites himself) spoken by Burgeon in Worlds Apart, perhaps one of the two or three most comprehensive concise expositions of Barfield's entire thought.
It is in association with the symbols which we call names that we build up, from childhood on, the coherent world of distinct shapes and objects which we call "nature." The "merest" sense-experience we can imagine ourselves having is also a process of formulation. Whatever else it is . . . the world that actually meets our senses is not a world of "things," which we are then invited to speculate on or experiment with. Any world which pure sensation--pure sensitivity to stimuli--could experience must be a mere plethora--what William James tried to suggest with his phrase "a blooming, buzzing confusion." Yet we never do in fact consciously experience such a world. We have converted the percepts into concepts, and moreover into systems of concepts, before we even know we have been hit by them. As far as our conscious experience is concerned, the perceptual world comes over its horizon already organized. But who has done the organizing? What are you going to call this preconscious organizing of perceptual experience, which gives us the world as we actually and consciously experience it? Coleridge called it "primary imagination." My friend Barfield called it "figuration." Langer, who has dealt with it much more fully and authoritatively, calls it "formulation." Both of them, and Cassirer, and many others, agree that it is the same activity as the activity which we call, when we are aware of it--thinking. (48)
See in particular "Thinking and Thought" (RCA 47-66), "Man, Thought and Nature" (RCA 223-40), "Thoughts and Thinking" (WCT 13-21), Worlds Apart, passim.
1Like most thinkers about thinking, Barfield distinguishes (for starters) between "perception" and "thinking":
    Perception is essentially a passive experience, something that happens to us; thinking is an active one, something that we do. Or if you don't like this distinction, because of refinements such as the 'intentionality' which some have detected (rightly I would say) in perception, or on the other hand because of the passivity of that uncontrolled type of thinking called 'reverie,' then thoughts are something that comes from within; perceptions something that comes from without. (HGH 11)
2In The Gay Science, Nietzsche makes a similar observation, placing the blame for our new style of thinking on America:
    The distinctive vice of the new world is already beginning to infect old Europe with its ferocity and is spreading a lack of spirituality like a blanket. Even now one is ashamed of resting, and prolonged reflection almost gives people a bad conscience. One thinks with a watch in one's hand, even as one eats one's midday meal while reading the latest news of the stock market; one lives as if one always might "miss out on something."
3The contraction of thinking today is, Barfield has noted, really "another aspect of the dying process, or rather it is the dying process, experienced from within. And what today we distinguish sharply from ourselves, and coldly investigate, as 'Nature,' is itself the end product of that process" (RCA 238).