 Williamson’s argument against the significance of the distinction between a priori and a posteriori justification rests on comparing how D and A (and other similar pairs of propositions like 4a and 4b in the examples above) are justified. He contends that they are both justified by relevant manipulations in imagination. But on traditional accounts of justification, D would be a paradigm case of some proposition that can only be a priori justified and A a paradigm case of some proposition that can only be a posteriori justified. Williamson seems happy to concede that there is some sort of difference (2013: 294–295, 296–297) between these two types of justification but to deny that it is a significant difference given the way that each type relies on manipulations in imagination. He might grant that the difference is like the difference between seeing something through a microscope or telescope as opposed to seeing something with the naked eye. There is a difference, but it is not a significant difference since these three types of seeing all rely on visual perceptions.