The part that specifies "which transitions are possible and which are not" (more precisely: which state results from a given state and input) -- known as the state-transition function [1] -- is a necessary component of the state machine definition. If you don't have that, you don't have a state machine in code; you just have a bunch of functions.

[1] https://en.wikipedia.org/wiki/Finite-state_machine#Mathemati...