Could anyone help me identify three words that are meaningful in Zen Buddhism in this poem??

“Past, Present, Future: Unattainable”
by Haikuin

Past, Present, Future: Unattainable
Yet clear as the moteless sky.
Late at night the stool's cold as iron,
But the moonlit window smells of plum.

Question

Could anyone help me identify three words that are meaningful in Zen Buddhism in this poem?? 

“Past, Present, Future: Unattainable” 
by Haikuin

Past, Present, Future: Unattainable 
Yet clear as the moteless sky. 
Late at night the stool's cold as iron, 
But the moonlit window smells of plum.

anonymous · Answer

I'm completely stumped. Could one maybe be present?

anonymous · Answer

You could easily argue that it's present, future, and clear.

anonymous · Answer

This is the whole poem?