Epicurus thought that he could trace the causes of human unhappiness to mistaken beliefs in his society, beliefs about the gods, the destiny of the soul, and the objects in life which are truly valuable. – A. A. Long
There are two equal and opposite errors that people make about beliefs. The first, that beliefs can move mountains. The second, that beliefs don’t count for much.
First, we must define beliefs correctly, and this is seldom done. A belief is a claim that is accepted as true, which then becomes a rule of action. A belief that is not acted on is not a belief but is mere mental assent.
Second, the claim does not need to be clearly articulated. Most are not. Rather they are assumptions that are just taken for granted. They are absorbed from family, friends, and society. They are very hard to detect.
Now Epicurus comes along and suggests that the causes of human unhappiness are mistaken beliefs in society. By unhappiness, he is talking about what we might call mental illnesses, and what the Buddhists would call suffering.
Stop and consider it for a second.
Your mistaken beliefs are making you sick.