 Ockham famously reduced the ten Aristotelian categories to two, substance and quality, treating the other eight as modes of signifying substances and qualities. For Crathorn, however, the entire Aristotelian system has to be revised. The human mind naturally knows nothing but qualities, and we cannot be certain that even they exist without appealing to the principle that God could not deceive us. Thinking and reasoning are of no help because, as we saw above, they are purely conventional.