What is the symbolism behind "poison" in days 4, 5, 6 of The Name of the Rose?

At the heart of the book The Name of the Rose by Umberto Eco is a controversy over medieval literary theory, a subject in which Eco himself is an academic expert. To understand how poison functions symbolically in the text requires delving into the conflict between Platonic literary theory, represented by the antagonist Jorge, and Aristotelian literary theory, which was gradually supplanting it in the period in which the novel is set.

At the center of their differences is a notion concerning the human response to art and literature. Plato believed that we imitate what is set before us. Thus he thought that when people read Homer, which tells tales of gods committing adultery and other immoral actions, or when people see pornographic paintings, that they will imitate what they see. This is still an argument made against explicit portrayals of violence and drug abuse in movies and video games, namely that we have a natural propensity to imitate what we experience. Jorge expresses this point of view when he condemns the gargoyles and illustrations of monsters that were common in Gothic art:

Before the eyes of monks intent on meditation, what is the meaning of those ridiculous grotesques, those monstrous shapes and shapely monsters? Those sordid apes? Those lions, those centaurs, those half-human creatures, with mouths in their bellies, with single feet, ears like sails?

For Jorge, grotesque imagery and laughter are a form of poison that pollutes the inner character of an audience. The literal poison he puts on the pages of the second volume of Aristotle's Poetics (a book that was, in fact, lost or destroyed in the Middle Ages), a volume that discusses comedy, was meant to kill people's bodies to avoid poisoning souls.

Aristotelian literary theory is based on the notion of "catharsis" or purgation. Aristotelians believe that by experiencing and reflecting upon terrible deeds and events, we can purge our souls of unwholesome emotions. This is, in part, based on an ancient medical notion that by taking small quantities of poisons that cause us to vomit, we can purge our bodies of more dangerous poisons and illnesses. 

Thus poison symbolically stands in for heretical ideas or images which can either pollute (according to Plato) or purify (according to Aristotle) the soul.