The problem of Philosophy in dealing with these rival aspects of the world of experience then becomes that of deciding whether either of them can be adopted as the truth in isolation from the other. Or if neither is the whole truth, we must ask ourselves in what way the world can be at once One and Many, how the characters of systematic unity and of indefinite variety can be consistently thought of as belonging to the same Reality. Is Reality, we have to ask, One or is it Many, and if it is both, how are the unity and multiplicity connected? The answers which different philosophical systems have given to this question may conveniently be classified under three general denominations. There are (1) the _Monistic_ views, which lay the principal stress upon the unity of the real, and tend to treat the aspect of plurality and variety as illusory, or at least as of secondary importance; (2) the various forms of _Pluralism_, according to which the variety and multiplicity of real beings is the primary fact, and their systematic unity either an illusion, or at any rate a subordinate aspect of their nature; (3) _Monadism_, which aims at harmonising the positions of the monist and pluralist, by treating the world as a multiplicity of really independent things or “monads,” which are somehow combined from without into a system. From this last point of view the plurality and the systematic unity are alike real and alike important for the understanding of the world, but are of different origin, the plurality being inherent in the things themselves, the unity external to them and coming from a foreign source. Within each of the three main types of theory there is, of course, room for the greatest divergence of view as to the special nature of the real. A monistic system may be purely materialistic, like that of Parmenides, who taught that the world is a single homogeneous solid sphere, or idealistic like that, _e.g._, of Schopenhauer; or again, it may treat mind and “matter” as “aspects” of a common reality. A pluralist or monadist, again, may conceive of each of his independent real things as a physical atom, as a soul of any degree of organisation, or even, in the fashion of some contemporary thought, as a person.