In the desert, people see mirages. Most people have probably seen a kind of mirage on the highway in summer. Way up ahead, there appears to be a large pool of water on the road. If we are to have any genuine knowledge, we have to be able to distinguish appearance from reality. There is no completely reliable way to so because we can always be mistaken.
Genuine knowledge is forever out of our grasp.
Of course, science provides a sound method for approximating truth. By scientific study we can determine what is most probable. But a necessary part of science is that every claim it makes is open to doubt. People used to believe that the earth was flat. We believe it is round.