"Cum Deus calculat et cogitationem exercet, fit mundus." This sentence, added in the margin of the short Dialogus dated 1677, 2 has long been one of the most debated passages of Leibniz's works. Undoubtedly the fact that Louis Couturat chose a shortened form of it ("Cum Deus calculat . . . fit mundus") as the opening motto for his fundamental text on Leibniz's logic 3 contributed to this. Ever since then, Leibniz's sentence has often been referred to and variously interpreted, both in support of and in opposition to the 'logicist' interpretation of the Leibnizian philosophical system, depending on whether the stress is placed on 'calculare' or 'cogitationem exercere'. The following pages are a brief discussion of some aspects of the Leibnizian conception of God's evaluation of possible worlds. The first part will consider the definition of perfection, as applied to possible worlds, the second part will investigate the possibility of using some texts on logical calculus to improve our understanding of the nature and functioning of God's calculus. Though some aspects of Couturat's interpretation are taken up, it is not the aim of this paper to repropose the opposition between 'logicist' and 'antilogicist' readings of Leibnizian philosophy. This opposition seems now to have become superceded in the critical debate. The possibility of approaching Leibniz's thought from different angles and perspectives is a direct consequence of the very articulation of this thought, and one of its major merits. We shall take as our starting point for this study some fundamental theses that, in our opinion, seem to stem directly from Leibniz's views on God's calculus, and we shall make a preliminary mention of them below. (1) First and foremost, the term 'calculus', as used by Leibniz, is to be understood in its strong sense. The subsequent reference to God's 'cogitare' is not to be seen as casting doubts on this: Leibniz often uses 'calculare' and 'cogitare' or 'ratiocinare' as synonyms, 4 he approvingly cites Hobbes' idea of thought as a form of calculus, s and clearly shows that he considers the direct relation between calculating and reasoning to be the basis for the construction of highly 'technical' systems of logical calculus. 6 (2) Moreover, Leibniz furnishes elements that, as well as being further proof of the 'strong' interpretation of God's calculus, also outline its specific nature: a calculus of maximization.