tion III addresses these issues through a modified spacetime formalism. The grounding of this formalism in the spacetime path approach is discussed in the appendix, making the connection to previous work in [24–26]. Once the formalism is established, Sec. IV describes the foundational concepts of systems and subsystems used in subsequent sections. The core of the paper comprises Sec. V, which addresses the topic of measurements, and Sec. VI, which address the topic of Born’s rule. Section VII then applies these concepts to the paradigmatic thought experiment of Schr¨ odinger’s cat. Finally, Sec. VIII presents some concluding remarks on the assumptions underlying Zurek’s envariance arguments in relation to the spacetime approach discussed here. Throughout, I will use a spacetime metric signature of ( − + ++) and take ℏ = c = 1. II. CONSISTENT HISTORIES The consistent histories approach to non-relativistic quantum mechanics assigns prob- abilities to quantum histories . Such a history is a sequence of quantum properties at a succession of times t$_{0}$ < t$_{1}$ < . . . < t$_{n}$ . At each time t$_{i}$ , the properties of interest are rep- resented by a set of projection operators ˆ P α$_{i}$ i , where the α$_{i}$ label different properties the system might have at time t$_{i}$ . These operators satisfy and Each possible history is then completely identified by a sequence of labels and the set of all such histories is known as a family of histories for the system. Now consider the time evolution of a non-relativistic quantum system from an initial state | Φ( t$_{0}$ ) 〉 under a generically time-dependent Hamiltonian operator ˆ H ( t ): 4 | Φ( t ) 〉 = T ( t, t$_{0}$ ) | Φ( t$_{0}$ ) 〉 , (2) α = ( α$_{1}$, α$_{2}$, . . . , α$_{n}$ ) ∑ α$_{i}$ ˆ P α$_{i}$ i = 1 (1) ˆ P α i ˆ P β i = δ$_{αβ}$ ˆ P α i