  Another theory is Mereological Nominalism, according to which the property of being scarlet is the aggregate of scarlet things, and for which something is scarlet in virtue of being a part of the aggregate of scarlet   things.[12]  An aggregate, or mereological sum, is a particular. But the theory faces a difficulty with so-called extensive properties like mass and shape. Not every part of the aggregate of square things is square since, for instance, not every sum of squares is itself square, and not every part of a square is itself square. So it is false that square things are square in virtue of being parts of the aggregate of square things.