 The United Kingdom is increasingly using units of volume according to the SI metric system, namely, the cubic meter and liter. Mathematically, the volume of a body may be defined by means of integral calculus. The amount of space occupied by an object or system is called the volume of the object or system. The volume of an object is equal to its mass divided by its average density (the term "average density" is used for an object that does not have uniform density). A one-dimensional object, such as a line in mathematics, or a two-dimensional object, such as a square, is assigned zero volume in three-dimensional space. The act also provided that "the defendant has the burden of proving the defense of insanity by clear and convincing evidence." The volume of a solid object is given a numerical value that quantifies the amount of three-dimensional space it occupies. Volume is sometimes distinguished from the capacity of a container. The volume of an object is one of the physical properties of the object.