Imagining is not oriented to the things of experience as remembering is. It is not its purpose to recall some single thing that was once experienced. The imagination can devise a world where the sky is green and trees are blue, where things fall up instead of down, rivers run backwards, etc. I mean that imagination may alter not only the concrete existence of particular things, but also the general types of experiences and the laws of nature. This free variation, however, has its limits. I can lend a thing any color I wish but it must have some color or other. I can constantly vary its shape but I cannot imagine it without any shape--otherwise it would no longer be a "thing." No more can I imagine a lion that is too unlionlike without it ceasing to be a lion.
The essence of things, what they are in themselves and what follows therefrom, sets bounds for the imagination (just as on the other hand the free variation of the world of experience leads to knowledge of its essential structure). Thus all intentional life, insofar as it constructs a world of things, turns out to be objectively bound.
Edith Stein, Potency and Act: Studies Toward a Philosophy of Being, Redmond, tr. ICS Publications (Washington, DC: 2009) p. 370.