"The Great War"
Between roughly 1925 and 1927, C. S. Lewis and Owen Barfield, both then in their late 20s, conducted a debate, precipitated by Barfield's new allegiance to Anthroposophy,  over metaphysics, epistemology, and the nature of the imagination.

Lionel Adey's monograph C. S. Lewis' "Great War" with Owen Barfield (1978), now back in print, meticulously reconstructs the war.

See also Stephen Thorson's essay "Knowing and Being in C. S. Lewis' Great War wth Owen Barfield."