THE ESSENTIALS OF SPIRITUAL UNITY (1918) describes the way Knox developed his thoughts on the Church. From the preface: "I began to be anxious about my position as an Anglican; I felt that I had no right to plunge into Catholicism (although I then held most of its doctrines) without going back over old ground and satisfying myself that I had not unduly neglected the claims of other denominations to a hearing.
Among other experiments in this direction, I began to write down some account of what I meant by "a Church." A Church I was determined to have, but it seemed to me it might clear my mind if I started with the bare idea and definition of a Church and followed out the implications of that idea, wherever (as Plato says) the argument should lead.
My method was not that of Plato, but that of Aristotle, at least in his Ethics. For Plato knows what he thinks beforehand, and his dialogue form is a literary artifice, but Aristotle seems (at any rate) to set out on no other basis than that of generally received ideas--What do we mean by "good"? What do we mean by "deliberate"? and so on--and, by whittling away the rival explanations that will not do, arrives in the end at the definition he wants. This nice slovenly method I adopted.
When I found myself (as usual) "up against" the Catholic system, I exchanged this experimentalist for an a priori method and began asking: If such and such a system of religious organization is the only tolerable kind of Church, how would such a Church (supposing it to exist) be likely to appear in the records of history? How much should we expect historical and geographical accidents to obscure, at first sight, the principles on which it was based?
The first part was begun as early as August 1915, but the work went on slowly and casually, as the mood took me, and the last part was never really finished--the last page or two I actually wrote in September 1917, just before I was received. I have let it stand as I wrote it, except for half a dozen incidental corrections which were suggested to me. I do not pretend that it is the way in which one ought to arrive at the idea of the Catholic Church; it is merely the way in which one soul did."