The four elementary operations of arithmetic are addition, subtraction, multiplication and division. Counting is the most basic concept of arithmetic. Counting in the most fundamental sense involves the set of numbers called the natural numbers (also called counting numbers for this reason). It is the ordered set of numbers {1, 2, 3,...}. Basic arithmetic generally takes place in this setting. When counting is done, a number is incremented from one member of the set to the next. (See Peano axioms and Counting in set theory for a higher-level discussion.)