John Locke published *An Essay Concerning Human Understanding* in late 1689 (the title page reads 1690), after working on it intermittently for nearly twenty years. The book is the systematic English answer to the Cartesian rationalist inheritance.

Locke's basic claim is simple, its consequences sprawling. The mind at birth is a blank tablet, *tabula rasa*. There are no innate ideas — no built-in idea of God, no innate moral principles, no innate logical axioms. All ideas come from two sources: *sensation* (impressions of external things) and *reflection* (the mind turning its attention onto its own operations). Complex ideas are constructions out of these simple ones.

The distinctions that follow from this simple starting point are what make Locke lasting. *Primary qualities* (shape, motion, number) belong to objects themselves; *secondary qualities* (colour, sound, smell) are powers in objects to produce effects in us. The distinction, aligned with the science of Galileo and Boyle, unsettles everyday realism: the red is not in the rose, but in the eye.

Locke's account of personal identity is perhaps his most original chapter. What makes someone the same person across time is not body or immaterial soul, but continuous consciousness and memory. If a prince's memories were transferred into a cobbler's body, who is who? The thought experiment is the starting point of every modern debate on identity — psychological continuity theories, teleportation puzzles, and beyond.

The cumulative effect of the *Essay* was to redirect philosophy. Berkeley, Hume, and Kant all respond to Locke directly. Enlightenment politics — if minds are blank slates, there are no innate inequalities — drew nourishment from this book. Today the cognitive sciences still argue with Locke whenever they ask what is 'innate' versus 'learned'.