Assumptions are great.
They save a lot of time and energy and make navigating daily life smoother.
example; we assume that we will arrive where we intend to go, so we don't need to consider a mechanical check of the car, or consult the public transport administration to see if everything is running ok.
You can imagine how bogged down it would get if we had to consider the details of every intention.
On the other hand, there are triggers that tell us that it's appropriate to put assumptions aside and check details. When a red light appears on the dashboard of the car, or when problems with public transport is detected.
There are those assumptions that have a more profound effect on quality of life.
The trigger for one of these is spiritual seeking. Searching for the 'meaning' of life. Dissatisfaction with happiness levels.
These triggers point to a really fundamental assumption that is universal. If everybody you know carries the same assumption, you might be forgiven for assuming that it is fact and not open to be re-considered.
When thinking of this one, the fact that the whole world once thought the earth was flat, says something...
Todays equivalent of the Flat Earth belief is the assumption that there is an actual self, that there is an I.
You don't need to be an astronomer to work this out. You can investigate it for yourself quite easily.