Plato draws a series of oppositions between knowledge and opinion. The central opposition is ontological. Knowledge, because it is unchanging and certain, must be of things which are rather than things which are becoming, or of the eternal rather than the temporary. Since the phenomena, things experienced with our senses, are constantly changing, knowledge cannot pertain to the phenomenal realm nor can it be derived from our senses, but instead, it must pertain to noumena (or the realm of the forms) and be obtained through reason. In some of the middle dialogues (especially Phaedo and Meno), Plato explores the possibility that the soul has perfect knowledge of the forms before descending into the body, whereupon it forgets, and that learning is therefore a process of recollection. This theory, however, is only present in a small group of dialogues and may represent an exploratory metaphor as opposed to an actual claim.