Rule one is embodied by the Schrödinger equation. The rule is essentially smooth, deterministic, and reversible. We hand it a quantum state, and it’ll tell us exactly what that state becomes at every future moment. This rule is, as far as we can tell, “exactly true”. We have successfully tested it to great precision.
Rule two is the measurement postulate, or as some refer to it, the so-called “collapse” postulate. When you measure a quantum system, its smoothly evolving superposition (see Rule one) abruptly jumps to a “definite outcome”, with probabilities given by the Born rule. (Explainer: each possible outcome carries a number called its “amplitude.” If we square that number, we get the probability of actually seeing that outcome. That squaring is the Born rule.) This rule is also, as far as we can tell, exactly true. Every experiment confirms it.
The trouble is measuring supplies energy. The canonical cat is alive or dead from that point on. Dig it?
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