Metaphysics (Wikipedia)

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Metaphysics is the branch of philosophy that studies the fundamental nature of reality. This includes the first principles of: being or existence, identity, change, space and time, cause and effect, necessity, actuality, and possibility.

Thief of Time (allthetropes.org) Thief of Time is the 26th Discworld novel and the fifth in the Death/Susan theme, although it also feels like something of a standalone.

There are metaphysical problems, which cannot be disposed of by declaring them meaningless. For, as I have repeatedly said, they are “beyond physics” indeed and demand an act of faith. We have to accept this fact to be honest. There are two objectionable types of believers: those who believe the incredible and those who believe that “belief” must be discarded and replaced by “the scientific method.” Max Born, Natural Philosophy of Cause and Chance (1964)