W. B. Yeats, Rejection of The Silver Tassie in a Letter to Sean OCasey (April 1928).
My dear Casey ... I had looked forward with great hope and excitement to reading your play, and not merely because of my admiration for your work, for I bore in mind that the Abbey owed its recent prosperity to you. If you had not brought us your plays just at that moment I doubt if it would now exist. I read the first act with admiration, I thought it was the best first act you had written, and told a friend that you had surpassed yourself The next night I read the second and third acts, and tonight I have read the fourth. I am sad and discouraged; you have no subject. You were interested in the Irish Civil War, and at every moment of those plays wrote out of your own amusement with life or your sense of its tragedy; you were excited, and we all caught your excitement; you were exasperated almost beyond endurance by what you had seen or heard, as a man is by what happens under his window, and you moved us as Swift moved his contemporaries.
But you are not interested in the great war; you never stood on its battlefields or walked its hospitals, and so write out of your opinions. You illustrate those opinions by a series of almost unrelated socrues, as you might in a leading article; there is no dominating character, no dominating action, neither psychological unity nor unity of action; and your great power of the past has been the creation of some unique character who dominated all about him and was himself a main impulse in some action that filled the play from beginning to end.
The mere greatness of the world war has thwarted you; it has refused  to become mere background, and obtrudes itself upon the stage as so much dead wood that will not bum with the dramatic fire. Dramatic action is a fire that most bum up everything but itself; there should be no room in a play for anything that does not belong to it; the whole history of the world most be reduced to wallpaper in front of which the characters must pose and speak.
Among the things that dramatic action most burn up are the authors opinions; while he is writing he has no business to know anything that is not a portion of that action. Do you suppose for one moment that Shakespeare educated Hamlet and King Lear by telling them what he thought and believed? As I see it, Hamlet and Lear educated Shakespeare, and I have no doubt that in the process of that education he found out that he was an altogether different man to what he thought himself, and had altogether different belief. A dramatist can help his characters to educate him by thinking and studying everything that gives them the language they are groping for through his hands and eyes, but the control must be theirs, and that is why the ancient philosophers thought a poet or dramatist Daimon-possessed.
This is a hateful letter to write, or rather to dictate - I am dictating to my wife -and all the more so, because I cannot advise you to amend the play. It is all too abstract, after the first act; the second act is an interesting technical experiment, but it is too long for the material; and after that there is nothing. I can imagine how you have toiled over this play. A good scenario writes itself, it puts words into the mouths of all its characters while we sleep, but a bad scenario exacts the most miserable toil. I see nothing for it but a new theme, something you have found and no newspaper writer has ever found. What business have we with anything but the unique?
Put the dogmatism of this letter down to splenetic age and forgive it.