 (a) In classical mechanics it is assumed that all variables of a system have always a value at every time. RQM, on the contrary, assumes that this is in not the case in nature in general (Heisenberg 1925; Kochen &amp; Specker 1967). Rather, this is only an approximation that holds when disregarding quantum phenomena. Physical variables acquire values only at some times, and have no value at the other times. Events, in other words, are discrete. This is the basic intuition that led Heisenberg to find the key to quantum mechanics in 1925; that is, there are questions that make sense in classical mechanics, but have no significance in nature. For instance, the question “What is the \(y\)-component of the spin of an electron when its \(z\)-component is \(\frac{1}{2} \hslash\)?” is meaningless: neither it admits an operational definition, nor is it needed for a realistic understanding of nature (see below, about realism). When does then a generic variable \(A\) of a system \(S\) acquire a value? When does an event happen? The RQM answer is: when and only when the system \(S\) interacts with a second system \(S'\) and the effect of the interaction on \(S'\) depends on the variable \(A\). In RQM, this is what it means for a variable to have a value.