Sextus Empiricus Pyrrhoniae Hypotyposes 2.14.200

[200] So likewise with an argument in the form ‟If it is day, it is light; but it is not light; therefore it is not day.” For from the fact that we do not observe day without light the hypothetical premiss ‟If it is day, it is light” might be considered to be valid; just as if, should day, let us suppose, at some time appear, without the appearance of light, the premiss would be said to be false; but, so far as concerns the axiomatic argument aforesaid, the non- existence of day when light is non-existent is proved by the premiss ‟If it is day, it is light,” so that each of these statements needs for its confirmation the secure grasp of the other in order thereby to become credible by means of circular reasoning.