A rational idea is an indirect description of the material world.
In chapter 18 of 30 Philosophers, it is defined in a footnote like this.
Empirical Ideas exist in the minds of beings and represent concrete objects and their universals that exist in the Material World; occupy space and time and can be interacted with directly or indirectly making them falsifiable.
An empirical idea is an idea that describes the material world directly. It is not merely a thought that feels right, a tradition we inherited, or a story that comforts us. It points to something in reality that can be observed, measured, tested, or verified.
In TST, empirical ideas are the strongest bridge between our minds and the material world. They are still ideas, not reality itself, but they are ideas that reality can check. A claim like “water boils at a certain temperature under certain conditions” can be tested. A claim like “germs cause disease” can be investigated. A claim like “the Earth orbits the Sun” can be supported by observation, measurement, and prediction.
That is what makes empirical ideas so important. They are disciplined by contact with the world. If an empirical idea fails when tested, we do not get to keep it just because we like it. We refine it, limit it, or abandon it.
In the Idea of Ideas, empirical ideas sit closest to reality. They do not give us absolute truth in our hands, but they give us our best tested descriptions of the material world.
