 The argument hinges on the idea that knowledge can’t be had by luck. Specifically, to know something, it must be that you couldn’t have been wrong very easily. Otherwise, though you might be right, it’s only by luck. For example, you might correctly guess that there are exactly 967 jellybeans in the jar on my desk, but even though you’re right, you just got lucky. You didn’t know there were 967 jellybeans, because there could easily have been 968 jellybeans without you noticing the difference.