Descartes for example supposed his experience while dreaming could be indistinguishable from his experience awake, so that no internal criterion could divide delusion from knowledge.

• 比方说，笛卡儿，当梦境不可能和他醒时的经验划分开来的时候，他假定自己还是有经验的，于是就没有什么内在的标准能够把错觉和知识分开。