This paper discusses ἐπιβολαὶ τῆς διανοίας, which later Epicureans are supposed to have elevated to a fourth criterion of truth to complement perceptions, preconceptions and feelings. By examining Epicurus’ extant writings, the paper distinguishes three different senses of the term: ‘thought in general’, ‘act of attention’ and ‘mental perception’. It is argued that only the sense ‘mental perception’ yields a plausible reading of ἐπιβολαί as a criterion of truth. The paper then turns to the textual evidence on ἐπιβολαί in later (...) authors. While the term ἐπιβολή is not used by Cicero, Lucretius and Philodemus in the sense of mental perception, it is argued that this still is the most plausible way of understanding ἐπιβολή as a criterion of truth. (shrink)