François-Marie Arouet, known as Voltaire, and Jean-Jaques Rousseau were two great French Enlightenment thinkers whose ideas had a profound impact on their era and on modern thought. Despite similarities in their values, over time the two come into conflict and became more or less enemies. This paper describes the development of their quarrel and identifies its possible sources through analysis of their correspondences and the philosophical differences espoused in their writings.