The Lettres Persanes, on which Montesquieu worked in leisurely fashion for perhaps ten years, reflect to a large extent the voluminous reading he must have done during this time. Their great variety of subjects implies numerous possible sources of inspiration, not all as yet indicated by Montesquieu scholars. It is my purpose to consider as possible sources of some ideas in the Lettres Persanes three authors of Montesquieu's epoch—Malebranche, Leibniz, and Shaftesbury. By the expression “possible sources” I mean that there are between Montesquieu and these three writers some relationships of ideas which may imply influence, since Montesquieu esteemed these writers and knew, or could have known, their works before 1721, the date of the publication of the Lettres Persanes. Two of these authors, Malebranche and Shaftesbury, were in fact greatly admired by Montesquieu. Ranking them as highly as Plato and Montaigne, he even calls these four writers “the four great poets.”