What defines the poetry of Yeats more than any other poet is his use of symbols to explore his chosen themes of aging and art and the impact of the Irish Civil War and World War I on life and society. Critics have identified that a common symbol he uses is that of spirals, particularly in his work of the 1920s and 1930s. What is common to the presentation of these symbols is how they are used to explore his particular themes. A spiral motion thus expresses simultaneously a reptition but also a progression, because at each repeated spiral you are retracing your steps whilst at the same time ascending slightly higher, making it new and different. This is particularly important in poems such as "The Winding Stair," because it allows Yeats to explore the process of aging, growth and identity, using this symbol as a governing metaphor for life.
At the same time, the symbol of the spiral in "The Second Coming" presents a very different outlook on life, as it indicates the gradual but inevitable disaster that Yeats felt was coming upon society as a result of Modernism. Note how this symbol is expressed in this poem:
Turning in the widening gyre
The falcon cannot hear the falconer;
Things fall apart; the centre cannot hold;
Mere anarchy is loosed upon the world...
The symbol of the spiral in this poem is a metaphysical representation of the "anarchy" that Yeats felt was about to be unleashed as a result of the moral disintegration that occurred during World War I and as a result of the Irish Civil War. A "gyre" is a circular or spiral turn, and Yeats uses this symbol to express his view of history, which he felt was drawing one age to an end, resulting in a "second coming" and anarchy as a result of the transition between one age and another. Symbols therefore are an incredibly important part of Yeats' poetry, even though his symbolism is somewhat abstruse, recondite, and difficult to penetrate.