Our knowledge as to physical objects depends throughout upon this possibility of general knowledge where no instance can be given. And the same applies to our knowledge of other people's minds, or of any other class of things of which no instance is known to us by acquaintance.
In chapter ten, Russell succinctly describes our capacity for a priori knowledge. He holds that our knowledge of the physical world is available only indirectly through an inference from acquainted sense-data. Our use of inference depends on the possibility that we can know something without direct experience.