Throughout the history of philosophy the concept of unity has presented a problem. What does it mean to say that the cosmos is one, that a thing is one, that an organism is one, that a nation is one, that mind and body are one, that knower and known are one? Exactly what is it that is denoted when unity is postulated of anything? And when two or more entities are conceived as subsisting in unity, exactly what is the relation between that unity on the one hand, and its correlative duality or plurality on the other?