Cicero's concept of natural law (lex naturae o lex naturalis) has been usually interpreted as a direct expression of Stoic philosophy. von Arnim has included Cicero's well known characterization of natural law (De re publica III xxii 33 and De legibus I vi18-19) in his collection of the Stoic fragments as the only passages explicitely related to natural law, even if Cicero never mentions the Stoics nor alludes to Stoic sources in them. Since then the belief in the Stoic filiation of Cicero's theory has become predominant. The circular argument at the basis of this hypothesis has not been remarked by most of the scholars. Further, there is usually a certain confusion in the different analysis that tend to make no difference between natural law (lex naturalis, lex naturae, ius naturae, ius naturales) and laws according to nature (leges secundum naturam). The paper clarifies this distinction that Cicero has carefully maintained and that differenciates the classic conception of law from the modern of natural law and later analyzes the relationship of natural law with mind (nous) and shows the Platonic origin of Cicero's theory and points to the differences with the Stoa.