For Barfield, the philosophy of René Descartes was almost an inevitability —foreordained by the evolution of consciousness:

The historical student of words and their meanings [and Barfield himself was, of course, just such a student] could almost predict, apart from any other source of knowledge, the appearance at about this time [the 17th century] of some philosopher who would do intellectually to the cosmos what Copernicus and Kepler had already done astronomically—that is, turn it inside out. And in Descartes, with his doctrine of “Cogito, ergo sum,” we do, in fact, find just such a philosopher. His influence was immense. Practically all philosophy since his day has worked outwards from the thinking self rather than inwards from the cosmos to the soul.
(History in English Words)

Though an historical creation, the Cartesian world view came to be seen as common sense, as central to what Allen Wheelis calls our “scheme of things.” Barfield was anxious to remind his contemporaries of both its temporality and its fallaciousness.

Not only what we think, but also the way we think is not necessarily the divinely appointed way for human beings to think, but is simply the way in which we have come to think, partly by persuasion and partly by the force of habit. We may come to see, instead, how the Cartesian diremption of mind from matter, though methodologically fruitful, is ontologically factitious. Mind and matter can be separated notionally, and then separately explored. But that does not mean they are separate. Separate exploration has brought unheard of gains in accuracy, but only to one side of the coin, only to the exploration of the outer world from without. The other side was first disparaged as so-called “occult qualities,” then ignored, then treated as nonexistent.
(“The Evolution Complex”)

Cartesianism has led to some bad intellectual habits:

Our habit of beginning, as it were, with space and time, as if they were existents, and then planting a number of objects in them, may be traceable to the Cartesian innovation. Whereas it would perhaps be possible to begin with the process itself—in this case the structural process—and look at the order of events, as it were, from their own point of view. We should then perhaps find that the relation between structure and space is reciprocal and that it is not the inevitable nature of our minds, but the Cartesian abstraction, which makes us find the notion of space without structure less absurd than the notion of structure without space.
(Unancestral Voice 178)