 And we do not have any demonstration by which we are able to persuade the opinion of those who might maintain that the earth, the water, the stars, and all the bodies which we see are but appearances and “species intentionales”, supposing God might use these species or accidents to make all that we see appear to us. For one is not able to say that one knows a single thing as one should—according to the laws and the notions which Aristotle and other philosophers give of science—if one cannot demonstrate that it is impossible that the reason which one supplies, or the thing which one proposes, be not true. That is enough to persuade those who use reflection that there is nothing certain in physics and that there are so few certain things, that it is difficult to propound any… (QI XVIII: 53, translated in Mace 1970: 24–25)