The philosophical problem of how you can know that you’re not dreaming has a name: "dream skepticism." There are various other scenarios, called "skeptical scenarios," which philosophers have used to challenge the assumption that we have experience of a world which is external to us. How can you know, for example, that you are not experiencing an illusion created by some evil demon who has decided to deceive you, and that the things that you seem to see and hear are not really there? This possibility was discussed by one of the philosophers mentioned in the lesson, René Descartes, and you can learn about it in another TED-Ed lesson. How can you know that you’re not actually just a brain sitting in a jar in a scientist’s laboratory being stimulated with electronic impulses so that you think you’re having all sorts of experiences, even though you’re actually just a brain sitting there in a jar? How can we know that we’re not all living in a computer simulation?

One thing which is striking about the problem of dream skepticism is that it has arisen in a number of different philosophical traditions in different places at different times in history. This happens sometimes in philosophy and in other disciplines: the same idea or very similar ideas occur to people in completely different contexts. As well as the seventeenth-century French philosopher, René Descartes, the philosophers mentioned in the lesson were the ancient Chinese philosopher, Zhuangzi, and the tenth- and eleventh-century Persian philosopher, Al-Ghazali. Another philosopher associated with dream skepticism is an ancient philosopher from the Indian Buddhist tradition named Vasubandhu. Philosophers are still working on the problem today.

Aside from the problem of dream skepticism, dreams are quite a mysterious phenomenon. Modern science has allowed us to learn a lot about dreams but there is still a lot that we don’t know. For example, we actually don’t even know why we dream. There are many theories about this, which you can learn about from this TED-Ed lesson.

If this is the first time you have encountered philosophy, you can probably see that philosophical thinking is quite distinctive. Here’s another great example of the way that philosophers think about things.