Your mind is filled with “reactive” thoughts with regards to your experience, and your reactions are based on your beliefs. Since you neither can, nor have, experienced everything in the world there is to experience, your thoughts about the largest part of what you hold to be true about life, come from others, who may or may not, have had personal experiences of their own with regards to those same thoughts.
The bulk of the ideas which people hold to be true about life, are totally “unsubstantiated” by any kind of “personal” experience, or fact. People have a tendency to just believe what others believe.
This is the system of beliefs which everyone holds dear, and clings so tightly to.