Eighteenth century melancholy moved a single bodily fluid's malfunction into the basic mechanism of the body. In other words, as Willis and others began to envision the complex relationship between the brain and nerves as the seat of the soul, a confusion of fluids or a "thickening of the blood" was no longer a simple imbalance; it was a spiritual condition. So, if melancholy was a medical condition connoting an acute and divided sensibility, the total breakdown of body and soul could be prefigured in melancholy and achieved (potentially) in madness.
But of course, this is the problem: the relationship between madness and melancholy can be viewed as similar, cumulative, or absolutely distinct. In the first case, we look at melancholy and madness as both participating in a similar, slightly skewed dialogue between the body and soul. One component is sensibly affected and begins to affect the other which in turn affects the first. Feeling falls into a feedback loop which can no longer discern origin or intensity of feeling. In the second case, melancholy is the beginning of or potential for madness; the melancholic individual is naturally more sensible, and becomes, if context allows, more and more sensible. The acuteness of feeling can increase and increase until the capacity for diverse experience gives way to painful intensity and madness. In these first two cases melancholy and madness are potentially good or bad. While madness or melancholy remove one from community, they give one the opportunity to comment from outside on that same community, inviting those inside to learn a new language with which to speak in. Madness then invites an oppressive, unfeeling society to restructure itself after the prophetic moment of sensibility.
In the third case, however, where we look at melancholy and madness as different, melancholy represents a capacity for infinite feeling; one can become more and more melancholic as long as development goes uninterrupted. Madness, on the other hand, comes to represent the radical break with sanity; a hard blow to the sensibility totally alters the spiritual and chemical composition of the individual, damaging the delicate balance of the juices and sending the body and mind into chaos. The figure for melancholy might be Yorick, whose love (or affected love) for crying leads him to court sadness and indulge his capacity for tears. Yorick (along with Cheyne, Richardson, and many others) is able to write melancholy into a socially accepted, even fashionable context where feeling at the expense of vigor is mandatory for an artist or cultivated individual. The figure for madness, perhaps, is the Clarissa of pages 890 to 898. That section of the novel is terrifying precisely because we are afraid Clarissa will never come back from insanity; we are afraid she will remain in "a private madhouse where nobody comes" (895). In this case, melancholy represents a socialized distemper, while madness dissolves into absolute solipsism.