An Enquiry Concerning Human Understanding
David Hume · 1748
"We never actually observe cause and effect itself, only one event constantly following another — so our belief in causation, and in the reliability of induction generally, is a habit of the mind formed by repeated experience, not something reason or observation can ultimately justify."
Hume pushes empiricism further than Locke, to a genuinely unsettling conclusion: even our most basic assumption about how the world works, that causes produce effects, isn't something we ever directly perceive. When you watch one billiard ball strike another, you see the first ball move, then contact, then the second ball move — you never actually see the causal power connecting them, only the constant conjunction of the two events happening in sequence, over and over.
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Meditations on First Philosophy
"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."