The sharpest of the British empiricists and the philosopher who, in Kant's famous phrase, woke him from his dogmatic slumber. Hume pushed John Locke's empiricism to its unsettling conclusions: if all knowledge comes from experience, our belief in causation is just habit — we see billiard balls strike and move, never "cause" itself. The self is a bundle of perceptions, not a substance. Induction has no rational foundation. And crucially for ethics: you cannot derive an ought from an is.
The last point is the famous "Hume's guillotine," and it frames most subsequent moral philosophy. Hume's own answer was that morality rests on sentiment — sympathy, approval, disapproval — not reason. Reason, he wrote, "is and ought only to be the slave of the passions." This naturalistic picture of ethics fed directly into his close friend Adam Smith's Theory of Moral Sentiments and, more distantly, into Jeremy Bentham's utilitarianism.
Politically Hume was a cautious Tory skeptic of The Social Contract theory: government, he argued, rests on habit, interest, and convention, not on any original agreement. He also wrote a bestselling History of England and quietly demolished the rational case for miracles — earning him enough enmity that the university chairs he wanted were closed to him. He died cheerfully unrepentant, to the dismay of his religious contemporaries.