The “Principle of Sufficient Reason” has already run into the buffers on several grounds. In the case of propositions within formal axiomatic systems, Gödel’s Incompleteness Theorem throws a huge spanner in the works with respect to this matter, courtesy of the fact that his theorem proves that there exists, for any sufficiently expressive formal axiomatic system, at least one proposition that cannot be proven true or false within that system. Indeed, Gödel’s proof of this constructed a proposition of the required nature for elementary number theory.

His proof was particularly subtle, and relies upon the fact that one can devise a mechanical procedure for converting the symbol strings of any given proposition into a natural number. This number is known as the Gödel Number for the proposition in question. For any given number n, we can construct (as Gödel did in his proof) a function S(n), which returns true if n is the Gödel number of a proposition provable within the formal system of interest (in his case, elementary number theory). Therefore ~S(n) returns true if the number n is not a Gödel number for a proposition provable within the formal system in question.

Gödel then proved that it was possible to choose a special number, b, such that b was the Gödel number of the proposition ~S(b). Which leads to a contradiction, unless [1] the formal system is ultimately inconsistent, or [2] the formal system is consistent, but the proposition in question is not provable within that system.

It has since been proven that the same restriction upon provability of propositions applies to any formal axiomatic system possessing at least the same expressive power as elementary number theory. At which point, the moment one constructs using Gödel’s method, a proposition P of the form given above, one is left with no other option, but to adopt P or ~P as an axiom of a new, extended formal system, which then falls into the same trap, and so on recursively ad infinitum.

As for concrete as opposed to abstract entities, well, quantum physics has pretty much tossed classical causality into the bin. The wave function for a given quantum system in Hilbert space may have multiple solutions for a given set of quantum operators, and ultimately only informs us of the probability of a given outcome, not whether that outcome will actually take place. I’ve already devoted space elsewhere to the matter of entanglement, how that is related to the commutator for quantum operators acting upon the wave function of a system, and how this places restrictions upon what information can be extracted via measurement of a system within which entangled operators are acting.

As a corollary, it is impossible to assign a “cause” in the classical sense, to certain quantum phenomena. Experimental tests of Bell’s Inequality pretty much seal the deal.