The Principle of Indifference says that we should assign equal probabilities to outcomes that are on par. Why? The thing to say is surely: "Well, there is no reason for one outcome to be more likely than another." But the equal probability of outcomes only follows from this remark if the Principle of Sufficient Reason holds so that when there is no reason for something, it doesn't happen. So it seems that the Principle of Indifference presupposes some version of the Principle of Sufficient Reason.