Russell uses this term to refer to how humans perceive objects that exist in reality. Russell argues that there is a distinct difference between appearance and reality. Humans are limited to their own subjective experience of the physical object and can never know the true nature of reality.

Empiricism is the philosophical theory that proposes that all knowledge is derived from sensory experience. Humans develop understanding through connections built over multiple experiences and sensory perceptions. Russell explains that both empirical knowledge and priori knowledge contribute to knowledge.

Epistemology is a branch of philosophy concerned with The Nature and Limits of Human Knowledge. Epistemology studies how humans gain knowledge, make sense of the world around them, and determine whether concepts are true or false. Russell develops a theory that is both intuitive and logical while pulling from various philosophical works in the field.