The belief that the universe is fundamentally a unity, that there is, in other words, some fundamental principle from which all the variety of nature and experience can be derived, has been entertained in some form or another by the majority of philosophers. It is also the presupposition of most religions. If we hold that the universe is really one, or really a unity, it will follow that there is a distinction between reality and appearance. For the universe certainly appears as a plurality; it certainly seems, that is to say, that there are many things in the universe, chairs and tables and people and numbers and thoughts about them, which are really different one from another. If, then, the universe is really one, this appearance of plurality will be in some sense an illusory appearance, and it will be necessary to explain how this illusory appearance arises.