But after observation and analysis, when you find that anything agrees with reason and is conducive to the good and benefit of one and all, then accept it and live up to it. Without observation and analysis, of course. I'm sure everyone will agree that it is important to have an open mind and be willing to question what one hears. The opinions of teachers, elders and people in general, and the information contained in textbooks, often represents the collective wisdom of millions of people which has evolved over centuries. All science and mathematics is developed based on a certain set of axioms which are accepted and not questioned.
Classical mechanics is based on Newton's Laws of Motion.
Even if one were to say that all scientific assumptions are justified by experimentation, observation and empirical evidence, there is still the restriction that the validity of all such experimentation is limited to what can be perceived by our senses. Once one starts questioning the validity of the way our senses perceive the world, all scientific theories come to nought. Which means that one has to start somewhere by assuming something on faith. Even if one does agree that nothing should be accepted without observation and analysis, it still holds that our mental processes are heavily influenced by inputs from our environment. In fact, if we try to reason out everything for ourselves, there is the danger that we might be influenced by our personal prejudices, and would not be perfectly objective despite our best intentions.