Proposition: Feeling, as depicted by Jung, is a rational function.
Definition: Rational, decision-making.
Distinction: A rational function is a property of conscious scrutiny, an irrational is a property of unconscious perception.
How does a rational function work?
We use rational functions to make decisions. Thinking relies on laws of reasoning or logic, yet Feeling relies on personal principles or what we call personal values. The outcome of attunement with principles for Thinking is confidence of what one has come to know. The outcome of this for Feeling is harmonization with oneself on a personal level and others, or simply put, personal sympathy. Personal sympathy seems to be a more intuitive rather than rational notion, however, Jung rightly labelled Feeling as a rational function because attunement with personal values is a prerequisite for personal sympathy. Or in other words, the more in line the Feeler is with his personal values, the easier it will be for him to be sympathetic. The two entities stand in direct proportion.
One can argue that Thinking is a more rational function than Feeling because it stays firmer to its principles, or in other words, it requires only logical soundness to function. Yet principles of Feeling are established by highly subjective and often mercurial personal sentiments of the person in question and other individuals surrounding him. My thesis is that Feeling is a rational function because the harmonizing intuition is not the starting point but only an entailment of the Feeler's attunment with conscious principles. The harmonizing intuition is only as analogous to Feeling as confidence and critical attitude is analogous to Thinking. To call Feeling an irrational function is no less absurd than calling Thinking an irrational one for the critical attitude and notion of self-competence.