To be a bit more precise, I am interested in Watson's description of the cellula imaginativa as a troubled mediator between external reality and the faculties of "judgment and memory" (9); "troubled" because the rational part of the mind seems to require images produced by the imagination in order to function at all, yet there is no way to be sure these images will adequately represent reality and the imagination may further muddy the waters by combining images haphazardly (as in dreams). Watson uses the example of John of Morigny's Liber florum to demonstrate the severity of this anxiety, as well as one ingenious scheme for managing it in the context of divination: "the Virgin's appearance in dreams must be carefully vetted for diabolical deception. [... E]ven true visions may contain a whiff of the diabolic, a mingling of truth and error" (28). That this kind of thinking was still possible in Measure for Measure's day may perhaps be established by the prominent place in Descartes' philosophy, a generation later, of the famous "evil genius" hypothesis (e.g., Meditations 22).
And the scenes of the play leading up to Angelo's temptation do indeeed introduce the notion of the shaky relationship between image and truth. Vincentio's appointment of Angelo as his deputy begins with the ironic (in retrospect) claim that "There is a character in thy life / That to th'observer doth thy history / Fully unfold" (1.1.28-30). The metaphor is of an open book, the sight of which produces reliably intelligible images in the mind of the "observer"; but Angelo answers with a less confident figuration: "Let there be some more test made of my mettle / Before so noble and so great a figure / Be stamped upon it" (1.1.49-51), introducing a coining motif that will recur several times in 2.4.
Angelo first retools it as a metaphor for Claudio's crime in begetting a bastard:
It were as good
To pardon him [. . .]
[. . .] as to remit
Their saucy sweetness that do coin heaven's image
In stamps that are forbid. (2.4.42-6)
But the appearance of the coining image that most clearly associates it with the imaginative faculty occurs in Isabella's response to Angelo's claim that "women are frail too" (2.4.125): "Ay, as the glasses where they view themselves, / Which are as easy broke as they make forms. / [...] / [...W]e are soft as our complexions are, / And credulous to false prints" (2.4.126-7, 130-1). The imagination here is figured both as a fragile mirror of the soul and as a faculty for imprinting images on the woman's "soft" judgment. It was this latter formulation that would prove so enduring among British empiricist philosophers, beginning with Hobbes: "There be also other imaginations that arise in men [...] from the great impression made in sense" (Leviathan 1.2.4).
The argument thus far I hope has established that the crucial "temptation scenes" of Measure for Measure are, in fact, concerned with a mental faculty akin to the medieval "imagination," which stamps images or forms onto the mind in a way that can be misleading and problematic. But the question remains, given the ease with which Measure for Measure's characters--and indeed those of early modern drama in general--are taken in by the imagination's false coinings: how is one to go about "vetting" its distressing productions?
Works Cited
Descartes, Rene. Meditations on First Philosophy. Discourse on Method and Meditations on First Philosophy. Trans. Donald A. Cress. 3rd ed. Indianapolis: Hackett, 1993. 46-105. Print.
Hobbes, Thomas. Leviathan. Ed. J. C. A. Gaskin. New York: Oxford UP, 1998. Print.
Shakespeare, William. Measure for Measure. Eds. Ivo Kamps and Karen Raber. Boston: Bedford/St. Martin's, 2004. Print.