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In his letter asking Count Ernst von Hessen-Rheinfels to communicate his summary of the Discourse on Metaphysics to Arnauld, Leibniz, eager to learn Arnauld’s opinion of his metaphysical theses, emphasizes the importance and the variety of the questions that he touches upon in the Discourse: grace, God’s concurrence with creatures, miracles, the cause of sin, the immortality of the soul, and so on. Yet, despite its links with all those topics, the letter does not mention the doctrine of the complete concept of an individual. Nevertheless, Arnauld immediately attacks Leibniz’s thesis on complete individual concepts, asking how such a doctrine will be able to avert the necessitarian consequences which it seems to involve. Arnauld’s challenge gives rise to a discussion ranking among the most profound debates into which Leibniz ever agreed to enter.