How did Bertrand Russell’s empiricism change during his career?

Bertrand Russell’s empiricism underwent significant shifts throughout his career, moving from an early acceptance of non-empirical knowledge to a more rigorous focus on logical analysis and, eventually, neutral monism.1

Early in his career, Russell admitted the existence of synthetic a priori knowledge and concepts of unobservable entities. However, influenced by his pupil Ludwig Wittgenstein, his perspective evolved. He became convinced that the truths of logic and mathematics are analytic and that logical analysis constitutes the essence of philosophy.1

During his empiricist phase, Russell analyzed concepts based on what one is "directly acquainted" with in experience, including both sense data and properties construed as universals. Later, in his neutral monist phase, he attempted to demonstrate that even the concepts of formal logic are ultimately empirical, suggesting that the experience supplying them could be introspective rather than purely sensory. These developments were foundational to the later emergence of logical positivism and the verification principle.1