X and M are left to be interpreted by the person who needs to provide for a just solution to a problem. One of the most prevalent formalists was Aristotle. He wrote that a just distribution of something is proportionate to the amount of a justice-making quality a person possesses. For instance, if Person A and Person B both have the same quantity of justice-making Characteristic M, then Person A and Person B deserve an equal distribution of the element in question. If Person A and Person B have different amounts of justice-making Characteristic M, then the person who possesses Characteristic M more should get more proportionally.
Formal justice is criticized because it fails to provide sufficient components for justice.
It provides a necessary component, but as many critics point out consistency is not enough. In genocide, all members of a certain ethnicity are killed, so in that way it is consistent. The formalist theories of justice are flawed in that they provide no reason why consistent killing by the government is immoral. To overcome this flaw, most would agree more substance and material must be added to the theories of justice to be viable, and that's where we get the material theories of justice.