As a contrasting idea or concept:
* Opposite: This is the most straightforward synonym, emphasizing the direct contrast.
* Counterpart: This suggests a close relationship between the two concepts, but with opposing characteristics.
* Foil: This is more literary, emphasizing how the two ideas highlight each other's differences.
* Inverse: This implies a specific reversal of the original concept.
* Converse: This emphasizes the logical relationship between the two ideas, where one is the opposite of the other.
* Negation: This highlights the complete rejection of the original concept.
As a literary device:
* Contrast: This is a broader term, referring to the general use of opposing ideas or concepts.
* Juxtaposition: This focuses on the close placement of the contrasting elements.
As a philosophical term:
* Dualism: This refers to the concept of two fundamental opposing forces or principles.
* Duality: This is a more general term for the existence of two contrasting aspects or qualities.
The best synonym for "antithesis" will depend on the specific context and the nuance you want to convey.