Underlying Barfield's epistemology is the concept of polarity: the belief that subjective and objective, matter and spirit, inner and outer are all part of a continuum and that all dualisms have their origin in man's increasingly interior experience of the outside world.

With Coleridge, Barfield would insist that "All the organs of sense are framed for a corresponding world of sense; and we have it. All the organs of spirit are framed for a corresponding world of spirit; although the latter organs are not developed in all alike" (quoted in RCA 18). This correspondence is polarity. Insisting that the "immaterial" be "perceived along with the material," polarity accepts as equally, "self evidently real" that "our inmost selves are part of a wider, inner world" and "our physical bodies are part of a wider physical world" (HGH 60). "It is really an illusion," Barfield shows in Romanticism Comes of Age, "to locate the break, or gap, between ourselves on the hand and nature on the other--as we normally do--at the surface of our skins. It is set much deeper inside us; and what we look on as the 'outside' world is in fact in large measure the working of our own sense organization, in which the outside world 'lives'" (208-209).

As epigraphs to Poetic Diction, Barfield quotes the following passages from Aristotle and Coleridge respectively, both of which testify to his own conception of polarity:

And there is an intellect which is of this kind by becoming all things, and there is another which is so by producing all things. . . . And this intellect is distinct, unaffected and unmixed, being in essence activity. . . . Actual knowledge is identical with its object. . . . (But we do not remember because this is unaffected, whereas the passive intellect is perishable), and without this thinks nothing. (De Anima, III, 5)
    . . . grant me a nature having two contrary forces, the one of which tends to expand infinitely, while the other strives to apprehend or find itself in this infinity, and I will cause the whole world of intelligences with the whole system of their representations to rise up before you." (BL, Chap. XIII)
Throughout his work he cites other testimonies of faith in polarity; for example, the following lines from a poem by George Rostrevor Hamilton:
    When hill, tree, cloud, those shadowy forms
    Ascending heaven are seen,
    Their mindless beauty I from far
    Admire, a gulf between;
    Yet in the untroubled river when
    Their true ideas I find,
    That river, joined in trance with me,
    Becomes my second mind. (quoted in SA 95)
And this passage from an essay by Edwin T. Land, the inventor of the Polaroid color process:
There really is no outside world and no inside world; there is just one world. In many ways the tree certainly does not exist in the physical sense without the observer. The tree does not exist for radio waves of a certain wavelength, nor does it exist for neutrinos. The tree exists as part and parcel of the interaction between that part of the cosmos and our part of the cosmos, namely the "We" that have evolved over many centuries to be a partner with the tree. (quoted in EC 12)
Polarity is, finally, the only path available to the human mind which leads to true universality.
The whole point of polarity is that the more opposition there is, the greater the unity. That is the mystery of it, and therefore you can't . . . as a human being, really become universal, except by way of polarity. You may disappear, cease to exist altogether, but you can't really acquire anything that could reasonably be called universal consciousness except by becoming still more individual than you are now. The more individual you become, if it is really individual and not just simply, self-centeredness, the more universal you are. (SP 19)
See in particular "The Philosophy of Samuel Taylor Coleridge" (RCA 144-63), What Coleridge Thought, passim, "Matter, Imagination, and Spirit" (RM 143-54).