1. Empiricism All knowledge of things in the world is a posteriori (that is, based ultimately on experience). Purely mental (i.e., a priori) operations of reason do not provide knowledge about the world.

2. John Locke (1632-1704): Representational Realism $ The mind at birth is a blank slate (tabula rasa): no ideas are innate Knowledge is based on experience of simple ideas (e.g., yellow) or complex ideas (lemon), relations (father), abstractions (humanity) We know about the world because our ideas of “primary qualities” (e.g., solidity, shape) represent things as they really are. Ideas of secondary qualities (e.g., colors) represent things only as they appear to us. Qualities are supported by (unknown) substances

3. George Berkeley (1685-1753) Primary qualities depend on the mind just as much as secondary qualities There is no way to show how primary qualities represent real external qualities or substances What we perceive is the real world, but the only things we perceive are our ideas. So the real world consists only of ideas and the minds that perceive them (including God). That is, the world’s existence consists in its being perceived (esse est percipi) in regular patterns

4. David Hume: Skepticism All knowledge of the world is a posteriori—based on and limited to sense impressions or copies of those impressions or creations of imagination (ideas) We have no impressions of self or causality (other than temporal priority & continguity, but no necessary connection)—only habit Notions of cause-effect and self are based on custom and natural inclination, not knowledge 1711-1776