 Material distinction is a necessary and sufficient criterion for real difference, traditionally conceived, whereas there is formal distinction if and only if there is one substance in number (i.e. material identity in the strict sense) and a multiplicity of formal principles with different descriptions instantiated by it. Paul therefore inverts the terms of the question in relation to what earlier approaches had done. By means of the formal distinction Duns Scotus and John Wyclif had tried to explain how it is possible to distinguish many different real aspects internal to the same individual substance (the passage is from one to many). On the contrary, Paul is attempting to reduce multiplicity to unity (the passage is from many to one). What Paul wants to account for is the way in which many different entities of a certain kind (i.e. of an incomplete and dependent mode of existence) can constitute one and the same substance in number.