The person who has the greatest ability to fool you is yourself. You know all your secrets, you know how you think. All of us have an insider position from which to fool ourselves.
That makes fooling yourself your greatest threat. I also believe that great success comes from cutting through illusions, seeing things as they are.
Newton and Leibniz both invented calculus at the same time. But there's a reason one of them is a household name and the other isn't.
When Newton published his theory of gravity, he included a disclaimer: "Fingnum non hypotheses". I feign no hypotheses.
Newton couldn't explain gravity. But he had a really excellent description of it. A description so good, it destroyed all the other descriptions.
Newton knew he was on to something big with the calculus, because it lined up with reality. He could predict the movement of the planets, the tides of the ocean, rays of light, cannonballs.
But he also knew what he didn't know: why. 300 years later and we're just starting to figure that out.
How great it is to know what you don't know, and admit it. How much that increases your understanding of what you do know.
Practice intellectual humility.