As in Walton's first letter (I:L1:3) enthusiasm is a charged word. In this period it was as often a pejorative as an honorific word: fanatics of whatever stripe were said to be enthusiastic. This was particularly true of religious fanatics and self-proclaimed prophets. Such figures claimed to be directly inspired by God, to feel the presence of God within (which is the root meaning of the word in Greek). In an ironic twist on this meaning Mary Shelley presents Victor as a self-styled Ezekiel, wrought up not by a storm sent by God but in a mental hurricane of his own making as he endeavors to usurp the function of the deity.
Johnson's 1755 Dictionary is highly suggestive in its implicit aversion to the idea of enthusiasm, defining it thus:
1. A vain belief of private revelation; a vain confidence of divine favour or communication.
2. Heat of imagination; violence of passion; confidence of opinion.
3. Elevation of fancy; exaltation of ideas.
If anything, Johnson is even more severe on the practitioner than on the concept, defining an enthuasist as:
1. One who vainly imagines a private revelation; one who has a vain confidence of his intercourse with God.
2. One of a hot imagination, or violent passions.
3. One of elevated fancy, or exalted ideas.
In, as it were, burying his third definition beneath the others, Johnson implies that such exaltation is spurious, ungrounded in reality. His dismissive "hot imagination" might similarly be taken as a synonymn for the "ardour" so conspicuously shared and honored by the protagonists of the novel. Quite clearly, the repeated use of "vain" and "vainly" is intended by Johnson to remind us of their substantive, which is "vanity."