A rational idea is an idea that works through reason. It may not describe the material world directly, but it can still be structured, coherent, and useful inside a framework. Logic is rational. Math is rational. Justice is rational. Democracy, socialism, and communism are rational ideas inside political frameworks.
This matters because not all truth comes from direct observation. Some ideas are built from relationships, definitions, implications, and consistency. If two plus two equals four, we do not need to go outside and find four rocks every time. The idea works because it is logically structured within a mathematical framework.
But rational ideas still need humility. A rational idea can be internally coherent and still fail when applied to the material world. A political system can sound elegant and still collapse in practice. A theory can be beautiful and still make bad predictions.
In TST, rational ideas are not dismissed just because they are abstract. They are part of human truth. But when they make claims about the material world, they must eventually answer to empirical reality.