When someone says “I know,” he/she must question the level of the knowing. Awareness/Consciousness is knowing, and there are so many levels of consciousness. A rock, a tree, an animal, a human has so many levels of unconsciousness, subconscious, asleep, distracted, unaware, etc., all the way up to superconscious and enlightenment, where you know all–so many levels between.
On our plane of consciousness, we can only say “I know” when and if this knowing causes a change in behavior in us. Meditation is where you find your existential reality–where you know you are not the mind.