Two opposing ideas, in one sentence and with parallel structure, meant to create a contrasting effect.
Example: "There are so many things that we wish we had done yesterday, so few that we feel like doing today."
- (Mignon McLaughlin, The Complete Neurotic's Notebook. Castle Books, 1981)
The effect is used to show the contrast in a way that provokes thought in the reader. In this case,
Many -- few
Yesterday -- Today
Antithesis, in this example, explains the vicious cycle of procrastination by emphasizing that we have many tasks we wish to complete, without the drive to get it done.