Yi Hwang's analysis rests on Mencius's doctrine that human nature is originally good. The Four Beginnings are innate moral capacities — natural tendencies of the heart-mind that, properly developed, become the four cardinal virtues. The sprout of compassion, when cultivated, becomes the virtue of humanity (ren); the sprout of shame becomes righteousness (yi); and so forth. Because these sprouts are expressions of li — the moral principle inherent in human nature — they are fundamentally oriented towards the good. Evil arises not from the Four Beginnings but from the Seven Emotions when they become unbalanced or are expressed to excess or deficiency.
The formula that Yi Hwang defended — "the Four Beginnings are the issuing of principle; the Seven Emotions are the issuing of vital force" — was challenged by his correspondent Ki Taesung on the grounds that principle never issues without vital force. Yi Hwang's mature response was nuanced: he did not claim that the Four Beginnings arise without qi, but that they are the product of li's own dynamic and that qi follows along without distorting them, whereas in the Seven Emotions qi takes the lead and li is along for the ride. The distinction is one of emphasis and origin rather than ontological separation: both li and qi are present in every psychological event, but their relationship varies.
For Yi Hwang, the practical upshot of the Four-Seven distinction is a programme of moral cultivation focused on the emotions. The sage does not eliminate the Seven Emotions — they are natural and, when expressed in due measure, perfectly appropriate. What the sage does is ensure that the Seven Emotions are guided by the Four Beginnings: that joy, anger, and sorrow are calibrated to what humanity, righteousness, and wisdom require. The method is twofold: reverence (kyŏng) — a sustained attentiveness that keeps the heart-mind clear — and ethical study and practice that give the Four Beginnings their proper expression in concrete situations.
The Four-Seven debate is the defining episode of Chosŏn-dynasty Neo-Confucian philosophy. Yi Hwang's position is presented in the Ten Diagrams in the seventh diagram, concerning the mind and human nature. The full debate correspondence is translated in Michael Kalton et al., The Four-Seven Debate (1994).