The Epicurean theory of knowledge is usually characterised by the following two features: knowledge must be subordinated to ethical ends; sensations and affections are sufficient guarantees for the knowledge of the true and the good. These two assessments are in themselves correct, but they do not suffice to convey the complexity of the subject. We try to restore it here by showing that Epicurean empiricism is not only based on sensations, but also on notions. The Epicurean conception of the origin of truth is rooted both in sensations and in the evidence of general notions. Because it builds a coherent scientific project, ancient Epicureanism intends to present itself as a rational empiricism, based on a methodical use of notions, under the condition of a constant agreement with experience. It will therefore be necessary to take into account the twofold objective of the epistemology of Epicurus and his successors: to provide a solid foundation for our scientific hypotheses; and to arrive at a method of judgment capable of offering a reassuring vision of the world and thus contributing to the pursuit of happiness. After an examination of the main sources (§ 2), we recall why and in what sense sensation constitutes a primary evidence, which allows us by contrast to define error as an additional movement of the mind, produced by the judgment (§ 3). We then examine (§ 4) the status of the first and obvious notions - the preconceptions or prolepses. These are in themselves self-evident, in the same way as sensations or the immediate affects of pleasure and pain, because of both their immediacy and their original relation to sensations. However, preconceptions are not only spontaneous productions and anticipations of the mind, or natural generalisations: they are also tools of scientific investigation and have a real methodological role, the different aspects of which are specified here. Among the other signs of concern for scientific coherence, we then mention the subtlety of the Epicurean approach to demonstration: the Epicureans do not reject it outright, but rather strive to reform its use (§ 5). At the same time, they subject judgement to a complex methodology, whose applications are not only practical, but also scientific (§ 6). The importance of preconceptions in Epicurean epistemology is therefore emphasised in conclusion (§7), while indicating what separates the latter from an exclusively positive conception of scientific knowledge.