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His lecture was entitled Yeats's lyric drought, 1903-1908. He began by telling us that Yeats wrote only three lyric poems - 38 lines - between 1903 and 1907. That's nice to know when a month passes by and you haven't even finished one new poem.
The usual reasons given for this "fallow period" include disappointment at the marriage of Maud Gonne and his preoccupation with the business of the Abbey Theatre, he did write plays during this period. James Pethica argued that the real reason had to do with the poet's shift in creative focus away from his use of folklore. This happened partly because he realised that he was not as good a scholar as others involved in the field - Synge, Hyde, Lady Gregory - and did not have the same application and empathy with the people who knew the folklore.
George Moore wrote of Yeats accompanying Lady Gregory on her folklore collecting expeditions but staying outside the cottage while she went inside, talked with the people and wrote down their stories which Yeats later used.
Yeats comes out of this period with a new attitude towards his writing and towards the people. His love poetry now is concerned with a "She" not a "You" as previously and he is more inclined to see himself as apart from the common man.
We, too, had good attendance once,
Hearers and hearteners of the work;
Aye, horsemen for companions,
Before the merchant and the clerk
Breathed on the world with timid breath.
(At Galway Races - from The Green Helmet and Other Poems. 1910)