Gottfried Wilhelm Leibniz developed a similar idea to Thomas Aquinas’ doctrine of variety, which is known as the principle of plenitude.  He argues that there must be diversity in that which changes.[1]  This change and diversity is what produces the specification and variety of simple substances.  This diversity must involve a multitude in the unity or in the simple.  For, since all natural change is produced by degrees, something changes and something remains.  As a result, there must be a plurality of properties and relations.[2]  The principle of plenitude entails absolutely every way that a world could be is a way that some world is and absolutely every way that a part of a world could be is a way that some part of some world is.[3]

The principle of plenitude has been used to argue against modal realism.  The principle is supposed to ensure that there are no gaps in logical space.  There is some real concrete universe for every way a world could be.  This entails that there may be a plurality of worlds that are on balance more bad than good.  Theistic modal realism entails that each possible world is a real concrete universe that a perfect being has actualized.  In the Leibnizian tradition, the principle entails at least some of the worlds are so bad that no perfect being could actualize them.[4]  Hence, Leibniz committed to this world being the best of all possible worlds.[5]  This is called the less-than-best problem.