René Descartes was one of the first to claim that all we really know is what is in our own consciousnesses, and that the whole external world is merely an idea or picture in our minds. Therefore, he claimed, it is possible to doubt the reality of the external world as consisting of real objects, and “I think, therefore I am” is the only assertion that cannot be doubted. Thus, Descartes can be considered an early epistemological idealist.
Descartes' student, Nicolas Malebranche, refined this theory to state that we only directly know internally the ideas in our mind; anything external is the result of God's operations, and all activity only appears to occur in the external world. This kind of Idealism led to the Pantheism of Spinoza.