skeptic; sceptic
Refers to a person who habitually doubts or questions accepted beliefs, often in philosophical, scientific, or everyday contexts.
He is a skeptic when it comes to paranormal phenomena.
A skeptic does not easily accept claims with insufficient evidence.
Compound of 懐疑 (かいぎ, 'doubt, skepticism') + 主義 (しゅぎ, 'principle, -ism') + 者 (しゃ, 'person'). The term follows a common pattern for forming agent nouns from abstract concepts.