Is the causal sequence that gave rise to the present state of affairs finite or infinite? One reason to prefer a finite causal sequence is its relative simplicity (an infinite series has infinitely more members than any finite causal chain). But another is that there cannot be maximal fundamentality within a infinite causal history. After all, for any cause c of the present state there is a prior cause c – 1 that is ontologically prior to c. Even the infinite series, taken as a whole, does not qualify, since the causes that form the series are ontological prior to the whole (similar to how the bricks of an infinite wall are ontologically prior to the wall that is formed by them).

But there is nothing ontologically prior to something that is maximally fundamental. A being that is maximally fundamental is ontological prior to all (other) things. It has thus reached the logical limit of fundamentality; it is logically impossible to be more fundamental than that. Maximal fundamentality can be reached in a finite causal past, that is, the first cause of the series could in principle be maximal fundamental.

But an infinite causal past does not (and cannot) have a first cause. Fundamentality will always be limited (non-maximal) in a world with an infinite causal past. But whence this limit in fundamentality? If it isn’t an arbitrary or brute limit, then what it is its explanation? This theoretical problem is avoided by a metaphysical theory that postulates a maximally fundamental first cause. We thus have a reason in addition to relative parsimony to prefer the theory that the present has a finite causal past.