According to Descartes, Leibniz, and Berkeley, one's own ideas are the only things one can know directly.

Descartes

A French philosopher, mathematician, and scientist who is considered one of the fathers of modern Western philosophy, famous for his statement "I think, therefore I am."

Leibniz

A German polymath and philosopher, who made significant contributions to mathematics, metaphysics, and logic, known for his optimism and the concept of monads.

Berkeley

An Irish philosopher of the early modern period known for his theory of immaterialism, which denied the existence of material substance in favor of the perception of such.