Meditations on First Philosophy
René Descartes · 1641
"By systematically doubting everything that can possibly be doubted, including the reliability of the senses, Descartes arrives at one thing that survives the doubt — the fact that he is thinking, and therefore exists, cogito ergo sum — and builds all further certain knowledge back up from that single indubitable foundation using reason alone."
Descartes sets out to find a foundation for knowledge that is completely certain, immune to even the most extreme skeptical challenge. His method is radical doubt: reject anything that can be doubted at all, even slightly, including the evidence of the senses, which sometimes deceive us, and even basic mathematical truths, by imagining a powerful deceiver could be tricking him about those too. What survives this maximal doubt is the act of doubting itself — the fact that something is doing the doubting proves that thing exists, whatever else turns out to be false.
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An Essay Concerning Human Understanding
"The mind at birth is a blank slate, tabula rasa, with no innate ideas — all knowledge, without exception, is built up from sensory experience and the mind's reflection on that experience."