The Principle of Contingent Causation: The Impossibility of Infinite Regress.

[Angrboda]

The problem is that simulteneity is required to avoid a first, for if there is a first from some reference frame, the rest are contingent from that perspective.

Why would that be the case if there was no causal link? And, in fact, it is quite easy in a relativistic scenario, to have multiple events, even ordered in some reference frames, that have no causal links.

I'm simply going by the common definition that contingent things do not exist in all possible worlds but necessary things do. Not that I have any love for any of these definitions as my general reaction to the notion of necessary existence is, "What does that even mean?"