We review the conceptual development of (true) concurrency and branching time starting from Petri nets and proceeding via Mazurkiewicz traces, pomsets, bisimulation, and event structures up to higher dimensional automata (HDAs), whose acyclic case may be identified with triadic event structures and triadic Chu spaces. Acyclic HDAs may be understood as the extension of Boolean logic with a third truth value expressing transition. We prove the necessity of such a third value under mild

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... about the nature of observable events, and show that the expansion of any complete Boolean basis L to L with a third literal a expressing a = forms an expressively complete basis for the representation of acyclic HDAs. The main contribution is a new event state × of cancellation, sibling to , serving to distinguish a(b + c) from ab + ac while simplifying the extensional definitions of termination A and sequence A B. We show that every HDAX (acyclic HDA with ×) is representable in the expansion of L to L × with a fourth literal a expressing a = ×. A more ambitious point of view we have been promoting in recent years puts time and information on a symmetric footing as the respective distance metrics of dual spaces whose points are respectively events and states. This view associates events with state changes or "information deltas" ∆I and states with periods or "time deltas" ∆T . Besides symmetrizing time and information, this viewpoint also leaves time itself symmetric, that is, unoriented. Strengthening these associations to causal relationships, with events causing ∆I and states caused by (or at least persisting over) ∆T, orients time to agree with our perception of causality. † Concomitantly the shift from active voice with † We orient cause to agree with our inate perception of the universe as seeking ever more likely states.