When Descartes in the First Meditation asked, 'How do I know that I am not being deceived by an evil demon?', his question was about science. It concerned the very possibility of putting questions to nature and receiving a truthful reply. This was something we take for granted, but the idea of Nature, as such, was an astounding discovery. Nature never deceives, although we often confuse ourselves not knowing the right questions to ask. The same question will always receive the same answer. That's the essence of the idea that a scientific experiment is always reproducible.
Descartes' proof that a God exists who is 'not a deceiver', was, in his eyes, not only proof that an external world exists, but also proof that the external world is a natural world. Scientific knowledge is possible, provided we use our (God-given) powers of judgement responsibly.
One thing that comes out of this is the idea that human beings are in dialogue with the world. Once philosophy has paved the way for science, human beings can freely put their questions in the hope and trust that they will never be lied to.
However, there is something else here too; a resonance which is easy to miss. Holed up in his stove room, Descartes is engaged with a monumental struggle with reality. Reality is either God or an evil demon, one or the other. Not for one moment does he seriously consider the solipsist alternative, that I, and I alone exist. (Hence, critics of Descartes who deploy Wittgenstein's private language argument miss the mark. There is no private language for Descartes. My experiences or ideas have a reality which goes beyond the face they present to me.)
But this raises a suspicion about my bare question mark. Am I secretly personifying it? And if God is out of the picture, once and for all, has the world become my evil demon?