Lucretius devotes some sections of his De rerum natura to the treatment of dreams. He is particularly interested in the phenomenon of dream deception, namely the belief that the dreamt object is the object itself. This psychological phenomenon has some both unwanted and philosophically interesting implications. Firstly, from an ethical point of view, it can stimulate uncontrolled worries and feelings in the dreamer that undermine his/her mental tranquility and health. Secondly, from a physiological perspective, it seems to reveal a failure (...) in the mechanism undergoing the formation of mental representation. Thirdly, from the epistemological point of view, it seems to compromise the veracity of mental representations. This paper aims to explain how Lucretius dealt with this phenomenon. In particular, it will show how he succeeded, on the one hand, to explain the nature and the origin of dream deception and of its emotional aspect, and, on the other hand, to preserve the informative contents of dreams and their correspondence to reality. For this purpose the present essay will address the following issues: which is the condition of the soul and of the mind while sleeping; what kind of mental activity is dreaming; which is the nature and the origin of the simulacra responsible for dreams; which is the contents of dreams and how dream deception comes about; and finally why and how dream deception emotionally effects the dreamer. (shrink)