  Leibniz's account of modality opens the way for him to present a distinctive kind of compatibilist theory of free will. For, according to Leibniz, Caesar's crossing of the Rubicon was a free act insofar as it spontaneously flowed from his own individual nature or was part of his complete concept and insofar as it represented Caesar's choice of what he perceived to be the best option for him at the time. This action was not necessary – and therefore contingent – because its contrary (for example, staying in Gaul) does not imply a contradiction. In saying that there is no contradiction, Leibniz means that either property or action – crossing the Rubicon or remaining in Gaul – could co-exist within a complete set of properties of a Caesar. But as we saw above, there are some properties that, were they changed, the individual would cease to be. Thus, for example, it is a fair bet that Caesar was not free to become a trout. Put differently, we can say that, while Caesar crossed the Rubicon in this world and thus began the Roman Civil Wars, there is another possible world in which a Caesar did not cross the Rubicon.  (Again, the “existence” of the other possible world is meant only to imply that there is some set of compossible essences that include the Caesar-counterpart who does not cross the Rubicon.) 