 In the case of proposition D, we might learn that one inch equals 2.54 centimeters, do the math and see that nine inches equals 22.86 centimeters, and thereby see that nine inches is greater than nineteen centimeters. There are other ways to be a priori justified in believing some proposition is true than by what Williamson calls the method of simulation. And those ways of justifying a proposition a priori are significantly different from a posteriori ways of justifying a proposition.