In his sermon “A Divine and Supernatural Light” (1734), American theologian and minister Jonathan Edwards makes a vital distinction between speculative knowledge of the good and affectional knowledge of the good; only the saint possesses the latter kind of knowledge “for God, who commanded the light to shine out of darkness, hath shined in [his] heart, to give the light of the knowledge of the glory of God in the face of Jesus Christ” (2 Cor. 4:6). Edwards argues:
There is a twofold understanding or knowledge of good, that God has made the mind of man capable of. The first, that which is merely speculative or notional: as when a person only speculatively judges, that anything is, which by the agreement of mankind, is called good or excellent, viz. that which is most to general advantage, and between which and a reward there is a suitableness; and the like. And the other is that which consists in the sense of the heart: as when there is a sense of the beauty, amiableness, or sweetness of a thing; so that the heart is sensible of pleasure and delight in the presence of the idea of it. In the former is exercised merely the speculative faculty, or the understanding strictly so called, or as spoken of in distinction from the will or disposition of the soul. In the latter the will, or inclination, or heart, are mainly concern’d.
Thus there is a difference between having an opinion that God is holy and gracious, and having a sense of the loveliness and beauty of that holiness and grace. There is a difference between having a rational judgment that honey is sweet, and having a sense of its sweetness. A man may have the former, that knows not how honey tastes; but a man can’t have the latter, unless he has an idea of the taste of honey in his mind. So there is a difference between believing that a person is beautiful, and having a sense of his beauty. The former may be obtain’d by hearsay, but the latter only by seeing the countenance. There is a wide difference between mere speculative, rational judging anything to be excellent, and having a sense of its sweetness, and beauty. The former rests only in the head, speculation only is concern’d in it; but the heart is concern’d in the latter. When the heart is sensible of the beauty and amiableness of a thing, it necessarily feels pleasure in the apprehension. It is implied in a person’s being heartily sensible of the loveliness of a thing, that the idea of it is sweet and pleasant to his soul; which is a far different thing from having a rational opinion that it is excellent.