A mathematical model about a sufficiently large set of natural phenomena is called an “ab initio model” or a “first principles model” if the starting point of the model cannot be further reduced to more fundamental concepts (at least not within the language of the considered natural phenomena). This video makes the point that this is what every field of science aims for:
Now formulate for yourself an answer to the following question, and post it to the forum underneath: “Why do we call quantum physics an ab initio model? Why do we call classical mechanics an ab initio model? Is classical electromagnetism an an ab initio model (and if it is one, what are its first principles)?” After having posted, you may read the contributions of your colleagues. If these make you to revise your own answer, then add a reply to your own post with your revised thoughts (don’t modify your initial post). As always, you are encouraged to reply to the posts of others. Why not thank the people that made you to revise your answer?
At this point, you’re done with the mandatory part of this topic. If you want to dig deeper, you find a few more optional videos and exercises here.