There is an oft-cited explanation that Edwards gives toward the beginning of Religious Affections.
The affections and passions are frequently spoken of as the same; and yet in the more common use of speech, there is in some respect a difference; and affection is a word that in its ordinary signification, seems to be something more extensive than passion, being used for all vigorous lively actings of the will or inclination; but passion for those that are more sudden, and whose effects on the animal spirits are more violent, and the mind more overpowered, and less in its own command. [p. 98]
But, he is clearer and more explicit later in the volume about their connection to the soul’s faculties. Edwards argues against the genuineness of internal motions produced by “external ideas,” that they are no part of a person’s “spiritual experience.” External ideas merely impact man by way of the imagination–“affections…founded on imaginations.” He admits, “These imaginations do oftentimes raise the carnal affections of men to an exceeding great height,” explaining here in a footnote:
And as the motions of our sense, and fancy, and passions, while our souls are in this mortal condition, sunk down deeply into the body, are many times more vigorous, and make stronger impressions upon us, than those of the higher powers of the soul, which are more subtle, and remote from these mixed animal perceptions: that devotion which is there seated, may seem to have more energy and life in it, than that which gently and with a more delicate kind of touch spreads itself upon the understanding, and from thence mildly derives itself through our wills and affections. But however the former may be more boisterous for a time, yet this is of a more consistent, spermatical and thriving nature. For that proceeding indeed from nothing but a sensual and fleshly apprehension of God and true happiness, is but of a flitting and fading nature, and as the sensible powers and faculties grow more languid, or the sun of divine light shines more brightly upon us, these earthly devotions, like our culinary fires, will abate their heat and fervor. But a true celestial warmth will never be extinguished, because it is of an immortal nature; and being once seated vitally in the souls of men, it will regulate and order all the motions of it in a due manner the natural heat, radicated in the hearts of living creatures, hath the dominion and economy of the whole body under it. True religion is no piece of artifice, it is no boiling up of our imaginative powers, nor the glowing heats of passion, though these are too often mistaken for it, when in our jugglings in religion we cast a mist before our own eyes: but it is a new nature, informing the souls of men; it is a Godlike frame of spirit, discovering itself most of all in serene and clear minds, in deep humility, meekness, self-denial, universal love to God and all true goodness, without partiality, and without hypocrisy, whereby we are taught to know God, and knowing him to love him, and conform ourselves as much as may be to all that perfection which shines in him.
Jonathan Edwards, Religious Affections (Part III: The Distinguishing Signs), 218 [Yale, 1959].