While contemporary epistemologists consider Cartesian scepticism as a menacing problematic, it seems that Plato scarcely had any Cartesian doubts about knowledge of the extemal world. In this paper I ask why Plato had this cavalier attitude towards Cartesian scepticism. A quick first explanation is that Plato never conceived the challenge of Cartesian scepticism or at least, if he did, he missed the potential threat to empirical knowledge that such a challenge poses. I argue against this explanation and offer an altemative, more plausible explanation.Very briefly, I claim that Plato grasped both the logical possibility of Cartesian scepticism and its potential threat but remained impervious because of his ontological epistemology. For Plato, the empirical world can hardly be an object of knowledge, just like a dream can hardly be an object of knowledge. But for Plato this is not really worrying because, necessarily, forms must exist and these constitute the truly real world and the tme object of knowledge. What is deeply worrying for Plato is that most people do not realize the 'dreaming' condition of the empirical world and need to be 'waken up' to the intelligible world of the forms by the philosophers-kings.