 The covering law presents a more delicate problem. While it is probably safe to say that no simple and entirely compelling argument has been given for assuming its general validity, Piron [1964, 1976] and others (e.g., Beltrametti and Cassinelli [1981] and Guz [1978]) have derived the covering law from assumptions about the way in which measurement results warrant inference from an initial state to a final state. Here is a brief sketch of how this argument goes. Suppose that there is some reasonable way to define, for an initial state \(q\) of the system, represented by an atom of the logic/property lattice \(L\), a final state \(\phi_p (q)\)—either another atom, or perhaps 0—conditional on the proposition \(p\) having been confirmed. Various arguments can be adduced suggesting that the only reasonable candidate for such a mapping is the Sasaki projection \(\phi_p : L \rightarrow L\), defined by