If I say "all Chinese are Buddhists," I can be refuted by the production of a Chinese Christian or Mohammedan. If I say "I believe that all Chinese are Buddhists," I cannot be refuted by any evidence from China, but only by evidence that I do not believe what I say ; for what I am asserting is only something about my own state of mind.
Source: Religion and Science, 1935, chapt. 9
More info.: https://russell-j.com/beginner/RS1935_09-080.HTM