Another of the Big Lies
This is a bromide uttered by pseudo-intellectuals (no offense intended, sir) looking for an excuse not to think. It's an attempt to make intelligence, thought, and mental effort an act of futility, something to be sneered at. "Look how dumb those smart people are! Let's laugh at them. They think they know something. Ha-ha!"
Yes, they think they know something. That's a whole lot better than thinking that nothing can be known.
Actually, there is a grain of thruth in the statement. As you learn more in your life, things get confused, blurred. You begin to see many different angles, ways of looking at things that are completely in opposition. Eventually, it looks like truth can't exist in a world like this.
And then you come out the other side.
There is order. There is reason in this world. You just have to get through that forest of doubt and confusion. When you are smart, you know what is true. Men like gods.