antithesis

antithesis, (from Greek antitheton, “opposition”), a figure of speech in which irreconcilable opposites or strongly contrasting ideas are placed in sharp juxtaposition and sustained tension, as in the saying “Art is long, and Time is fleeting.”

The opposing clauses, phrases, or sentences are roughly equal in length and balanced in contiguous grammatical structures.

The world will little note nor long remember
what we say here, but it can never forget what
they did here.

In poetry, the effect of antithesis is often one of tragic irony or reversal.

Saddled and bridled
And booted rade he;
A plume in his helmet,
A sword at his knee;
But toom [empty] cam’ his saddle
A’ bloody to see,
O hame cam’ his gude horse
But never cam’ he!