If "notion" means idea or concept:
* Reality: This contrasts with the abstract nature of a notion.
* Fact: This emphasizes something concrete and proven.
* Ignorance: This suggests a lack of understanding or knowledge.
* Unknowing: This emphasizes a lack of awareness.
If "notion" means a vague or fleeting feeling:
* Certainty: This suggests a strong belief or conviction.
* Conviction: This implies a strong and settled opinion.
* Knowledge: This signifies a deeper understanding or awareness.
If "notion" means a sudden impulse or whim:
* Plan: This suggests a deliberate and structured approach.
* Reason: This implies a logical and thought-out decision.
* Purpose: This emphasizes a clear goal or objective.
The best antonym will depend on the specific context in which you are using the word "notion."