that N can be taken to denote M by virtue
of the qualities that they share, namely, P^1, P^2, P^3, P^4. 

The situation can be diagrammed as follows:
                                                                   
|                   P^1   P^2         P^3   P^4                    
|                    o     o           o     o                     
|                     \*    \         /    */|                     
|                      \ *   \       /   * / |                     
|                     