Many philosophers throughout history had, arguably, been tempted to reduce two of these kinds of belief and knowledge to the other one: René Descartes and David Hume thought that the only knowledge that people start with is self-knowledge. Some logical positivists (and some would say Wittgenstein, or Wilfrid Sellars) held that people start with beliefs only about the external world. (Arguably, Friedrich Schelling and Emmanuel Levinas held that people start with beliefs only about other people.) It is not possible, on Davidson's view, for a person to have only one of the three kinds of mental content; anyone who has beliefs of one of the kinds must have beliefs of the other two kinds.