In the sentence; the rational is rewarded. rational is an adjective. why?

Rational is defined as an adjective that describes something being reasonable or related to the application of reason.

Explanation:

The rational is a noun phrase that is made with an adjective that works as a noun in this sentence which does not lose it real meaning in the context, being the description of "rational" as quality and a subject, using its grammatical function in the sentence as a specific uncountable singular noun