Muslim philosophers have provided some standards to classify quiddative, philosophical, and logical concepts (for example, whether they are sensible or not; whether they exist through mental or external existence or both; whether they occur directly to the mind or are found through making comparisons). None of such standards, however, is complete. Author of the present article says that universal concepts are, at first, divided into two categories:
1- Those which occur to the mind;
2- Those which do not occur to the mind
Then, the second category is divided into those which express quiddities of things and are devoted to a certain group and those which do not refer to the quiddity (of things) and are not specified to a certain group but rather include all or most kinds, and each of them has its own divisions.
 . quiddative: of or pertaining to the quiddity or essence of a person or thing (New Shorter Oxford Dictionary).