Hugo Kołłątaj’s treatise On dreams, or my refl ections on the reveries of nighttime reveries (1796) is an attempt at analyzing and classifying various dream theories and at analyzing and interpreting his own dreams. He takes the rationalist view of dreams, which, following Aristotle’s naturalistic philosophy, Hippocrates’ humours and the modern mechanistic theories, looks for physiological, medical and mnemic explanations of this phenomenon. The aim of his article is to position Kołłątaj’s treatise, a notable product of the Polish Enlightenment, against the background of all these naturalistic theories as well as to identify the points where he shares their approach and those that are distinctly his own. What makes the comparison more complex is the cross-genre nature of the work: it combines the elements of a philosophical treatise on dreams, a catalogue of ‘curious dreams’ and an autobiographical essay.