The approach of first principles has been pursued in the development and history of physics. Ever since the establishment of the Standard Model of particle physics in 1970s, the idea of going after theory of everything has become popular as the latest approach of first principles among theoretical physicists for unifying all particles and interactions. However, we seem to live in a dynamic world as indicated, e.g., since the discovery of an expanding Universe and it is definitely at odds with the static picture of an ultimate unified theory for physics.

The dynamic picture tells us that the time reversal symmetry has to be broken and it has to be the first (broken) symmetry. Whatever first principles we propose have to be able to naturally break this symmetry first in the very beginning. And there is no reason why the current 4-dimensional spacetime, in particular, its dimensions can’t be dynamic. It is probably more natural to consider that spacetime has evolved in a dimension-by-dimension way.

First of all, we propose and summarize the three first principles as follows:

- A measurable finite physical world is assumed.
- The quantum version of the variation principle in terms of Feynman’s path integral formalism is applied.
- Spacetime emerges via dimensional phase transitions (i.e., first time dimension and then space dimensions got inflated).

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