Idealism is the epistemological doctrine that nothing can be directly known outside of the minds of thinking beings. Or in an alternative stronger form, it is the metaphysical doctrine that nothing exists apart from minds and the "contents" of minds. In modern Western philosophy, the epistemological doctrine begins as a core tenet of Descartes – that what is in the mind is known more reliably than what is known through the senses. The first prominent modern Western idealist in the metaphysical sense was George Berkeley. Berkeley argued that there is no deep distinction between mental states, such as feeling pain, and the ideas about so-called "external" things, that appear to us through the senses. There is no real distinction, in this view, between certain sensations of heat and light that we experience, which lead us to believe in the external existence of a fire, and the fire itself.