What you sense is not the same as what actually exists

Russell opens with a deceptively simple example: look at a table. Its color, apparent shape, and texture change depending on lighting, angle, and who is looking at it, yet we assume there is one stable table underneath these varying impressions. He calls the immediate contents of perception — the color patch you see, the texture you feel — "sense-data," and distinguishes them sharply from the physical object itself, which we never perceive directly.

This distinction matters because it reveals that our confidence in physical objects is actually an inference built on top of sense-data, not a direct perception of matter itself. We see colors and shapes; we infer tables.

Russell isn't arguing that tables don't exist, but that the relationship between what we immediately experience and what actually exists independently of us is far less direct than ordinary language suggests, and philosophy needs to examine that gap carefully rather than assume it away.

Takeaway: what feels like direct perception of the world is already an interpretation built from more basic sensory data.