Essentially the claim that nothing can be known and that our senses lie all the time, that reason tends to favor our desires. Some people claim that and say they maintain a matter of suspending judgment on just about everything.
They call it Pyrrhonism and try to apply that non judging attitude to all of life. It claims that much of strife is based on human belief and opinions. That when we value what is good we suffer if we don’t have it and struggle to hold it when we do, I’m guessing valuing something as bad works the same way. So by maintaining an attitude of permanent indecision you “free your mind” from worry and find tranquility. Seems somewhat like Buddhism and that religion is pretty large. But I have to wonder how sound that is and whether or not it is practical? It has some points to it though, our senses are easily fooled so why believe them? Reason tends to be influenced by our desires and emotions. Can what we get from such things really be called knowledge?