When Anselm attempts to ontologically demonstrate the existence of God, he establishes a clear distinction between existence in understanding and reality. For something to exist in understanding, there must be a clearly defined concept for said object, however for it to exist in reality it must exist and comprehend and also possess the quality of existence. Therefore, whenever you describe an object, you assume that it exists and then continue to describe its attributes. However, if one presupposes that existence itself is a quality, this makes the presupposed existence of the described object redundant or contradictory, depending on whether the object possesses the quality of existence. For example, when you say "dragons don't exist", you assume that dragons exist by mentioning them in conversation, but then continue to disprove their existence. This seems contradictory, however, as far as we know, "dragons don't exist is true