While Volpone says "yes" to every single pleasure he can find—and pursues those pleasures vigorously—Celia is defined by her self-denial. This makes her a perfect foil for Volpone, since her self-restraint exposes his complete lack thereof, no more clearly than in Volpone's attempted seduction of her. The turning point of the play comes when she says "no" to Volpone's advances, thus denying him the lascivious pleasures he describes in his seduction speech. Celia seems willing to do anything to avoid dishonor, and this makes her character flat and predictable, too ready to sacrifice herself to be believable. Her willingness to subject herself to Corvino's harsh dictates and abuse may make her seem more weak than strong. But she has an inner moral sense, (even if it is dictated by seventeenth-century conventions on femininity) indicated by the fact that she refuses Volpone against her husband's express wishes. The fact that Jonson sides with her can be seen in his decision to put one of the strongest statements of the play's thesis in her mouth: "Whither, whither / Is shame fled human breasts? Is that, which ever was a cause for life , /Now placed beneath the basest circumstance? / And modesty an exile made, for money?" Jonson again chooses a name with symbolic meaning for Celia: it derives from the Latin word caelum, meaning "sky" or "heaven."
Read more about the social expectations for women in Renaissance England.