Locke argued in "An Essay Concerning Human Understanding" (1689) that the mind begins as a blank slate (tabula rasa) and that all ideas derive from sensory experience and reflection, rejecting innate ideas. In the "Two Treatises of Government" (1689) he argued that legitimate political authority arises only from the consent of the governed, instituted to protect natural rights to life, liberty, and property, and that a government violating this trust may justly be resisted.
Intellectual setting
Locke wrote the "Two Treatises" partly to justify resistance to arbitrary monarchy in the period surrounding the Glorious Revolution of 1688, and his empiricist epistemology in the "Essay" responded directly to Cartesian rationalism's appeal to innate ideas.
Key ideas
Tabula rasa and empiricist epistemology
Locke argued that the mind has no innate ideas or principles; all knowledge originates in experience, arriving either through the senses (sensation) or through the mind's awareness of its own operations (reflection).
Government by consent and natural rights
Locke argued that legitimate political authority rests on the consent of the governed, instituted to protect pre-political natural rights to life, liberty, and property, and that a government that betrays this trust forfeits its authority and may be lawfully resisted.
Legacy
Locke is a founding figure of British empiricism, continued by Berkeley and Hume, and the "Two Treatises" directly influenced the American Declaration of Independence and the broader liberal constitutional tradition of government by consent.