“ Man's imagination, which whether thinking or dreaming, takes the forms of an innumerable number of things, appears to other men's senses, as it were embodied in the semblance of some animal. ”
 Thomas Aquinas, Summa Theologica (1274). copy citation

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“for just as he can from the air form a body of any form and shape, and assume it so as to appear in it visibly: so, in the same way he can clothe any corporeal thing with any corporeal form, so as to appear therein. This is what Augustine says (De Civ. Dei xviii, 18) : "Man's imagination, which whether thinking or dreaming, takes the forms of an innumerable number of things, appears to other men's senses, as it were embodied in the semblance of some animal." This not to be understood as though the imagination itself or the images formed therein were identified with that which appears embodied to the senses of another man: but that the demon, who forms an image in a man's imagination, can offer the same picture to another man's senses.” source