How we change what others think, feel, believe and do
Antithesis is the use of an opposite in order to highlight a point.
It has been said we are good, but I say we are bad.
In many ways, the old are young.
She is very intelligent -- far from stupid.
Antithesis uses the contrast principle to draw attention to something. When an opposite to what might be expected is given, it is usually a surprise and hence causes the listener to slow down and figure out the puzzle presented.
Antithesis may simply include the negative or may be done as a form of repetition, stating both the positive and criticized negative case.
Argument is often described as thesis, antithesis and synthesis where, briefly, an idea is put forward (thesis), it is debated (antithesis) and the idea is consequently improved (synthesis).
The principle of antithesis may also be used more broadly, for example where a character in a play acts in contrary ways, or where two characters have diametrically opposite positions, thus highlighting each other.
Litotes, where someone might say 'I am not unhappy' is a form of antithesis as 'unhappy' is the opposite of the intended 'happy'.