Definition extracted with permission from Simon, Fritz, et al, Family Process, Inc.: Language of Family Therapy: A Systemic Vocabulary and Source Book (Family Process Press Series)
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According to the theory of Logical Types, proposed by Alfred North Whithead and Bertrand Russell (1910 - 1913), one must distinguish between a class (set) and the elements of the class. A statement that refers to a class manifests a higher level of abstraction - in other words, is of a higher logical type - than does a statement that refers to the elements of a class or set. This distinction is of particular importance when two statements are so presented that it can not be determined from the outset whether reference is being made to the class as a whole or to an element of the class. The word "man" can refer to an individual being; it can also refer to a class, the class of all human beings. In the latter case, the concept of "man" is located at a higher level of abstraction and corresponds to a higher logical type.