We can, I think, see enthusiasm as the undifferentiated extreme of sensibility. In medical terms, enthusiasm occurs as the animal spirits move faster and faster; as one approaches the height of fear, passion, melancholy, etc., the organs respond to the increasing flow of juices; the heart beats more quickly and with more intensity, the lungs need to take in more air. So when any feeling reaches a certain dangerous intensity, it becomes enthusiasm. In Mackenzie's article in The Lounger, he refers to the danger of enthusiasm as the substitution of "certain impulses and feelings of what may be called a visionary kind, in the place of real practical duties." The intensity of feeling, the visionary, religious moment threatens to homogenize any sort of feeling into pure enthusiasm. If the sentimental project includes the reading of bodies and feelings in place of a language under suspicion, the enthusiast is an aphasiac. In Collins' "Ode to Pity," for instance, the poet's Pity engenders not pity at all, but a "wild enthusiast heat." The constructive potential of an externally directed impulse is compromised as the feeling loses identity and collapses into blank, though warm enthusiasm.
Of course, not every response to enthusiasm was as negative; Hugh Blair praises the "barbaric" Ossian for the "fire and enthusiasm of the most early times," and Hannah More, in an essay that otherwise criticizes sentimental fictions, praises enthusiasm: "And enthusiasm is so far from being disagreeable, that a portion of it is perhaps necessary in an engaging woman. But it must be the enthusiasm of the heart, not of the senses." More prefers the enthusiasm of the heart, suggesting perhaps that enthusiasm, when radiating outwards from within, is preferable to a sensible enthusiasm, in which case, external stimuli affect the juices which in turn affect the heart. It is, perhaps, better to be affecting than affected.