Historians usually consult letters by artists in order to verify and to interpret works of art. These letters contain basic information about technical, social and psychological aspects. As personal documents, they have always enjoyed a particular esteem among biographers and collectors. On the other hand, historians have often suggested that artists are the melancholic outcasts of society. But even a brief glance at the available correspondence proves that the majority of letters do not support this diagnosis. When artists mention their own physical constitution, they apologise for delays or request further financial support. Moreover, the letter is subject to literary codes according to which medical issues are adopted as a metaphor for more general problems. The French classical painter Nicolas Poussin, on whom the present analysis is based, displayed a particular interest in questions of style and literacy. His writings should then be regarded as complex "compositions" that stand comparison with his artistic oeuvre rather than mere informal messages to his readers. Indeed, correspondences between artists and their patrons, colleagues and friends form a "genre" in their own right that can be drawn upon as a reliable source for research in medical history.