&gt; It&#x27;s actually not true to say that Schrodinger&#x27;s equation, or F=ma, cannot be derived.<p>I don&#x27;t think I said it couldn&#x27;t be derived. I said there wasn&#x27;t a &quot;fundamental reason why the Schrödinger equation is X&quot;. Using Lagrangians to derive it then begs the question &quot;but why must electrons obey the Lagrangian?&quot;.<p>&gt; This is because in order to derive something, you need to first start from a set of assumptions. You can pick any assumptions you want, that is your privilege, as it is mine. If you want F=ma to be an assumption, you can go on and derive things from that.<p>Well, F = ma was a bad example, I grant you. It&#x27;s actually a definition of what a force is, nothing more. But it&#x27;s relevant, because it has a similar purpose (except in a different field of physics). A better example of a fundamental law would be action-reaction or something.<p>But yes, you&#x27;re free to pick any assumption you like. But if you assume X, which came about because of assumption Y, it shouldn&#x27;t be a shock that X can be used to derive Y. At that moment, they are just different notations for the same assumption. You might argue (and hell, I might even agree) that Lagrangians are so much more mathematically pleasing, so make a better assumption. But that doesn&#x27;t change the fact that you&#x27;re dressing up assumption Y as assumption X.<p>&gt; But truly, F=ma is not the most general assumption in theoretical physics.<p>&gt; If you look in any decent book on classical mechanics, you will find that F=ma in fact is derived from a much more general principle, the Lagrangian.<p>The Lagrangian is great for solving many problems. But it is definitely not more general than Newtonian mechanics. It can&#x27;t deal with friction, or quite a few other non-conservative forces. On the plus side, solving oscillating systems is much easier. And it&#x27;s so much nicer when not using Cartesian coordinates.<p>&gt; That is the same Lagrangian that is the subject of the OP, which is used to derive the Schrodinger equation; and it&#x27;s not just a coincidence.<p>While this is correct, Lagrangian mechanics refers to concepts such as energy which are defined from forces. In particular, Lagrangian&#x27;s deal with systems with only conservative forces (but total energy may change with time). So it&#x27;s really a circular argument to say &quot;this concept can be derived from this even more abstract concept, which was actually defined from the first concept and at the end of the day is a guess.&quot;<p>There&#x27;s nothing wrong with guessing fundamental laws. That&#x27;s how the scientific method works, after all. You ask a question, guess the answer, predict what your guess would imply and test your predictions.