 To know something, it seems you must have some justification for believing it. For example, your knowledge that Socrates taught Plato is based on testimony and textual sources handed down through the years. But how do you know these testimonies and texts are reliable sources? Presumably this knowledge is itself based on some further justification—various experiences with these sources, their agreement with each other, with other things you’ve observed independently, and so on. But the basis of this knowledge too can be challenged. How do you know that these sources even say what you think they say, or that they even exist—maybe every experience you’ve had reading The Apology has been a mirage or a delusion.