This is the prime example of a vacuously true but completely useless statement. It's exactly equivalent to the toddler's insight that you can always ask another

This is the prime example of a vacuously true but completely useless statement. It's exactly equivalent to the toddler's insight that you can always ask another why. Yes you can always ask "but how do you know?". But so what? What does that teach you about the nature of our knowledge of the world? Nothing of substance.