Imagination, as any magician will know, is not a matter of fantasy and the idle creations of the mind. Despite its denigration as "childish", as the wilful enactment of fallacy in the playground of the mind, Imagination is one of the key weapons in the magician's arsenal, and without it there would be no vessel in which to pour the power of the magician's Will. Imagination puts us in touch with a more primal element of our being, one that is exemplified in the child, unconditioned by the social limitations imposed upon it by adults who have been drained of this essential ability. It is Imagination that is tapped into by artists and writers, and so too does the magician partake of this art, to result in the creative process that is magic.
But to practice magic, both the Imagination and the Will must be called into action, for they are co-equal in the work; they are a symbiotic process, each empowering and complimenting the other. For the Will is Force, while the Imagination is Form; the Will the representative of the Fires of Spirit, and the Imagination the representative of the Masses of Matter.
The Will unaided can send forth a current, but this energy will dissipate or catapult chaotically about if there is no vessel to contain it, nor medium to channel it.
The Imagination unaided can create an image, a Tzelem, a vessel, a subject in which the current of the Will can operate, yet it can do nothing if it is not thus vitalised and directed, being, as it were, an inanimate golem, or a clay Adam bereft of the vitalising Breath of Life.
Thus, just as Mercy and Severity must be brought together in mystic marriage, and just as the Tree of Life tells the tale of the long voyage of the Light through the Twin-Houses of Force and Form, so too must the Imagination and Will be brought together, the one creating the vessel and the other filling it and setting it assail. When both are properly trained and made to work in harmony with each other, then is magic made.
Note that the above does not contain the examples (or illustrations) contained in the Flying Roll, which can be found, along with Frater Resurgam's original wording of these thoughts on the Imagination and Will, and Westcott's remarks thereon, here.