The scarcity
Objectivity is the constraint, and there is very little of it available.
Set out the questions a trading system has to answer and sort them by whether two competent people looking at the same evidence would agree. Is this a valid setup — no. Is the trend intact — no. Should this branch be used here — no. Is the market in expansion — arguably, with a defined method, but the method has parameters. How far is current equity below its highest realised value — yes, unambiguously, forever, with no method to argue about. That question is almost alone in its category, and the system's response is to build its non-negotiable layer on it and leave the contestable questions to discretion, where they belong. The gate is not the most important thing MARS measures. It is the thing MARS can measure without argument.
The loop runs every cycle and terminates the same way each time. Nothing in it takes an opinion as an input, which is what allows it to run identically on a good day and a bad one.

