Aristotle argues that geometrical objects can not exist substantially in Metaphysics 13, 2. Platonists are those who claim that geometrical objects are in sense objects or that geometrical objects exists independently on sense objects. Aristotle argues that geometrical objects are in sense objects materially(hulikōs). I mean 'materially' to be 'incidentally'(sumbebekōs) and 'potentially'(dunamei). Geometrical objects exist in sense objects not substantially but incidentally. They can not be changed per se(kath' hauto). Sense objects must be changed directly. When they are divided or combinded with other objects, geometrical objects can be changed indirectly. The potentiality of geometrical objects is realized by intelligence through the process of abstractions. Geometrical objects don't have actuality(entelecheia) in this sensible world. They have a indivisble formal aspect and a divisible material aspect. Material aspect is quantitative, but formal aspect geometrical objects have is boundary. If the geometrical object has n-dimensions, the boundaries of geometrical objects have n-1 dimensions. The boundary of solid is surface, surface is line, line is point. Since Aristotle claims that the three-dimensional solid is perfect object, the solid itself can not be a boundary. In addition, while surface and line have both quantitative and boundary aspect, point itself just have a boundary aspect. Focusing on the boundary aspect of geometrical objects, we can obtain the clues to the difficulties about infinity, place, time, and continuity in Aristotle's natural philosophy.