In the treatise On divination through sleep, Aristotle intends to refute the thesis of a divine origin of dreams which seem to announce future events. To do this, he does not simply support a different thesis. He also denounces the epistemological weakness of divinatory practice by proposing an alternative scientific model: a method of classification of dreams and division by exhaustion, which excludes inspired dreams. This method leads to underline the irreducible character of chance facts and to admit the inexplicable character of a great number of ‘true’ dreams. The notion of coincidence (sumptôma) plays an essential role on this point, generally neglected. This role is both descriptive and critical: it allows us to characterize the class of dreams whose veridical dimension is absolutely fortuitous, and it denounces the common illusion that links them to real events. From this point of view, chapter 1 of De divinatione makes a modest but specific contribution to the theory of physical modalities.