PHY.Preface:3.1 - Construct and constrain the account

PHY.4 begins while the response law is still unknown. Recover the participants, transform the relevant situation, and derive the restrictions supplied by symmetry, dimensions, balances and dissipation where their physical grounds hold. The remaining unknown function or state directs further work.

PHY.5 chooses the physical detail needed at the requested scale and duration. An omitted state can leave memory or fluctuations; a fast response can matter during a transient. Its result tells mathematical reduction what must be retained or bounded.

PHY.6 combines physical participants, exchanges, response laws and compatible preparation into an evolution. A balance alone can leave a response undetermined. Locating that missing contribution is useful before any solver is selected.

PHY.7 offers another construction when a physically justified variational principle applies. The interactions, allowed comparisons and boundary freedoms determine what is varied. Mathematical variation then derives a consequence. A direct balance remains sufficient in many cases; applying an action formalism to every problem adds work without a corresponding gain.

PHY.8 constructs collective predictions from physically admissible alternatives and their weighting grounds. It retains correlations, preparation and the difference between sampling a distribution and following physical evolution. A distribution or sufficient bound can remain useful when a particular moment or equilibrium approximation is unavailable.