Webster's "The Duchess of Malfi" is a play that relies on the temperaments of characters to propel the action and plot. The heavy emphasis placed on the importance of character development is blatantly apparent in the use of the humoral theory to develop each character. Each of the four main characters was created with a disposition based on one of the four humors. The basis of the theory is that an imbalance, abnormality or access of any one humor within the body can cause adverse physiological and psychological consequences. Webster, with this rationale in mind, creates actions and reactions for each character in relation to the balance of humors within their body and in accordance with the humor their character was based off of. The theory of humors was established to account for the mental state of individuals; mental stability was a sign of the internal balance of a person's body fluids, while mental illness was a sign of an access or abnormality of body fluids. .

             Webster is clearly establishing this correlation through the behaviors of each character in the play. Since the internal disposition of a person can affect the state of their mental health, the cause of an imbalance or access of the humors becomes important. Although the theory of humors claims many causes for the imbalance of body fluids, Webster seems to bypass traditional explanations and associates the makeup of a person's humors as a result of a person's moral core. The balance and harmony of humors is a direct result of a characters actions and conduct. Morally sound behaviour leads to greater balance of humors, which in turn leads to stable mental health. In comparison any morally ambiguous or inappropriate behaviour can cause a disruption in the humoral equilibrium of a person, affecting the mental state of an individual. .

             The actions and behaviors of the four main characters in "The Duchess of Malfi" will be analyzed and assessed according to their moral value, this will than be compared to the characters psychological health and stability in order to show the clear correlation that Webster's is trying to make between morality and mental well-being.