Experience and the limits of understanding › Objections and Responses

A difficulty for the reader is deciding exactly what the blank-paper metaphor commits Locke to. His account includes active mental operations; an objection to a completely passive mind would therefore miss part of the theory. Consider a general triangle. Any pictured triangle has particular features, but the general idea must apply across differences in angle or proportion. Locke describes the difficulty himself in Book IV, chapter VII: his general triangle is neither one particular kind nor another. The example makes abstraction a problem to explain, rather than an operation we should take for granted. George Berkeley challenges this account in the Introduction to his *Principles of Human Knowledge*. He denies that he can form the abstract image Locke describes. A particular idea can instead represent other particular cases without becoming an impossible image that combines incompatible features. This objection concerns how general thought works; it does not deny that general reasoning is possible. Another question concerns necessary truths. If the materials of thought come from experience, must every truth be established by repeated observation? Locke's account says no: knowledge concerns perceived relations between ideas, and demonstration can establish relations through intermediate steps. The origin of an idea and the justification of a judgment are different questions in his system. This is why "all knowledge comes from experience" needs care as a summary. Locke gives an experiential origin for ideas while distinguishing intuitive, demonstrative, and sensitive knowledge. Reading the slogan as a claim that every proof is an empirical generalization would erase the distinctions in Book IV.