This paper justifies the inference of physical probabilities from symmetries. I supply some examples of important and correct inferences of this variety. Two explanations of such inferences—an explanation based on the principle of indifference and a proposal due to Poincaré and Reichenbach—are considered and rejected. I conclude with my own account, in which the inferences in question are shown to be warranted a posteriori, provided that they are based on symmetries in the mechanisms of chance setups.