There are many important quotes from the play Purgatory by Yeats. Consider this statement from the Boy:

The floor is gone, the windows gone,
And where there should be roof there's sky,
And here's a bit of an egg-shell thrown
Out of a jackdaw's nest.

At the beginning of the short play, the Old Man and the Boy—his son—come upon what used to be a grand house. The Old Man explains that the house used to be his. The son sees a shell of the house without floors or windows or a roof. However, the Old Man can see his mother's ghost in the windows of the house. He hears hoof beats that signal that his father is returning home to impregnate his mother, the act that would cause his mother's death: she died giving birth to her son (the Old Man). His mother's ghost has to continually relive this act.

However, the Boy cannot see or hear this. He only sees a bit of eggshell thrown from a bird's nest. This eggshell could be seen to symbolize the shell that his father's house has become. It is also a symbol of the boy's impending death, as the bird has thrown its young out of the house in the same way that the boy's father will kill him.

Had loved the house, had loved all
The intricate passages of the house,
But he killed the house; to kill a house
Where great men grew up, married, died,
I here declare a capital offence.

In this passage, the Old Man speaks about the house and the way in which his father loved it. The house seems like a living thing. Its passages are like its veins, leading to its heart. The Old Man's father once set the house on fire when he was drunk. The man likens the fire to a murder. He believes his father killed the house as if it were a person. The house is not simply a thing but has a deep meaning and holds importance to the Old Man. The Old Man then killed his father to punish his father for this offense.

Hoof beats! Dear God
How quickly it returns—beat—beat—
Her mind cannot hold up that dream.
Twice a murderer and all for nothing,
And she must animate that dead night
Not once but many times!
O God!
Release my mother's soul from its dream!

The Old Man kills the Boy, his son, in an attempt to prevent his mother's ghost from having to relive the return of her husband and from having to re-experience her impregnation and consequently her death. However, though the Old Man has killed both his father and his son, he cannot free his mother's ghost from her nightmare. She is forever trapped in purgatory, unable to have her soul released. The Old Man begs God to release her soul, but it is in vain.

Yeats suggests that we cannot escape fate and history. Instead, we are forever trapped in a cycle of reliving the nightmares of the past.