Occam's Razor: the simplest explanation is usually right

William of Ockham, a 14th-century Franciscan friar, articulated the principle that 'entities are not to be multiplied beyond necessity' — in modern usage: given two explanations that equally fit the evidence, prefer the simpler one. Occam's Razor is not a logical proof but a methodological heuristic; it has guided scientific theorising from Copernicus (a heliocentric model is simpler than Ptolemy's epicycles) to modern physics (prefer unified theories over baroque patchwork). It has limits — reality is sometimes genuinely complex — but as default practice it has served the scientific project well.

Source: William of Ockham, Summa Logicae (c. 1323)