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Leibniz rejected Locke's claim that the mind was initially devoid of ideas, like a blank sheet of paper, because this would make new minds identical, but separate, a possibility ruled out by his Principle of the Identity of Indiscernibles.[[CiteRef::Look (2017)]] Although he allowed that contingent truths might be learned with the assistance of the senses, logically necessary principles, like the truths of pure mathematics, logic, and some areas of metaphysics and ethics could not come from the senses because no number of specific experiences could demonstrate their necessity. [[CiteRef::Look (2017)]] Therefore, he concluded that, "the proof of them can only come from inner principles, which are described as innate". [[CiteRef::Leibniz(2017a)|p. 3]] To explain why everyone doesn't have access to these innate ideas, he wrote that "It would indeed be wrong to think that we can easily read these eternal laws of reason in the soul...without effort or inquiry; but it is enough that they can be discovered inside us if we give them our attention: the senses provide the prompt, and the results of experiments also serve to corroborate reason, rather as checking procedures in arithmetic help us to avoid errors of calculation in long chains of reasoning". [[CiteRef::Leibniz(2017a)|p. 3]] Leibniz's criticisms of Locke touched off a prolonged debate between empiricists, who maintained, with Locke, that all knowledge derives from experience, and rationalists like Leibniz, who maintained that some knowledge is derived by means other than experience, and must therefore be innate. [[CiteRef::Markie (20172015)]]