In practice, we are usually concerned with the quantities of the members of a class, and to define classes we abstract. The set of all the objects on the picture has a quantity, but the objects belong to the set as long as they lay within the picture borders. Elephants, however, belong to the class of elephants because they are elephants.

Every book on the picture is different, and it belongs to a class of the same name books. To think of the quantity of books - any books - we have to generalize. Similarly, we can add apples to oranges, but only if we consider them fruits. Multiplying, we produce a new class. Tracking classes is utterly important for using arithmetic.

Every students is unique, and it belongs to a class of its own. Speaking about the quantity of students, we leave behind their personalities.

The previous and this page outline several lessons what can be played out using various household objects.

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