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Imagination is a faculty of the unreasoning part of the soul. It is through the organs of sense that it is brought into action, and it is spoken of as sensation. And further, what is imagined and perceived is that which comes within the scope of the faculty of imagination and sensation. For example, the sense of sight is the visual faculty itself, but the object of sight is that which comes within the scope of the sense of sight, such as a stone or any other such object. Further, an imagination is an affection of the unreasoning part of the soul which is occasioned by some object acting upon the sensation. But an appearance is an empty affection of the unreasoning part of the soul, not occasioned by any object acting upon the sensation. Moreover the organ of imagination is the anterior ventricle of the brain.
St John Damascene
An Exact Exposition of the Orthodox Faith, BOOK II CHAPTER XVII,Concerning Imagination.