Aristotle believed some people were 'natural slaves' — and this influenced Western law for 2,000 years

In his Politics (c. 350 BC), Aristotle argued that some people are 'natural slaves' by constitution — they lack the deliberative rational faculty needed for full autonomy and are thus suited by nature to serve those with reason. He used this to justify the institution of slavery in ancient Greece. The argument was revived by Spanish philosophers in the 16th century to justify the conquest of the Americas: indigenous peoples, they argued, were Aristotle's natural slaves. The theory shaped colonial law for three centuries. It is perhaps the most consequential philosophical error in history.

Source: Aristotle, Politics, Book I (c. 350 BC); Davis, D.B., The Problem of Slavery in Western Culture (1966)