How does introspection count as experience for some empiricists?

For many empiricists, introspection—often termed "reflection"—is considered a valid source of experience alongside sensory perception. While empiricism is primarily associated with knowledge derived from the stimulation of sense organs, many philosophers in this tradition maintain that the mind’s awareness of its own internal operations constitutes a form of empirical data.12

In this view, mental states such as being in pain, feeling frightened, or the act of thinking itself are considered "immediate experiences". Philosophers like John Locke argued that while sense experience provides knowledge of the external world, reflection provides the mind with materials for knowledge by making one aware of its own internal processes. These mental states are sometimes described metaphorically as being present to an "inner sense".312

Consequently, for these empiricists, introspection is not a form of a priori reasoning but an observational method used to discover the laws governing the mind. Even in later developments, such as the work of Bertrand Russell, introspection was sometimes utilized to supply the experience necessary to ground concepts, including those in formal logic.345