Focusing on the method of prediction:
* Skeptic: Someone who doubts or disbelieves predictions.
* Rationalist: Someone who relies on reason and logic, not divination.
* Scientist: Someone who uses empirical evidence and the scientific method.
Focusing on the accuracy of predictions:
* Prophecy denier: Someone who rejects the validity of prophecies.
* Non-believer: Someone who doesn't believe in predictions.
* Realist: Someone who focuses on practicalities and avoids wishful thinking.
Focusing on the role of the person:
* Debunker: Someone who exposes false claims and superstitions.
* Critic: Someone who analyzes and evaluates claims with a critical eye.
* Fact-checker: Someone who verifies the accuracy of information.
The best antonym for "soothsayer" depends on the specific context and what aspect of the term you want to contrast.