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Locke's main thesis is that the mind of a newborn is a blank slate (tabula rasa) and that all ideas are developed from experience. Book I of the Essay is devoted to an attack on nativism or the doctrine of innate ideas. Locke allowed that some ideas are in the mind from an early age, but argued that such ideas are furnished by the senses starting in the womb: for instance, differences between colors or tastes. If we have a universal understanding of a concept like sweetness, it is not because this is an innate idea, but because we are all exposed to sweet tastes at an early age.
Along these lines, Locke also argued that people have no innate principles. Locke contended that innate principles would rely upon innate ideas, which do not exist. One of Locke's fundamental arguments against innate ideas is the very fact that there is no truth to which all people attest. He took the time to argue against a number of propositions that rationalists offer as universally accepted truth, for instance the principle of identity, pointing out that at the very least children and idiots are often unaware of these propositions.
