Red Tractors Green Tractors

My Tractor Tom's Tractor

Figure 2-4. Example of grouping and individualization.

All tractors used in a farm can be grouped in one category regardless of their brand, color, horsepower, and year of production, and be represented by one concept such as Tractors. In case we need to use one of them with a certain horsepower, then we need to browse the set of tractors and find that particular individual that satisfies our needs. In this case, we have individualized one element of the set based on some particular criterion. When we say Tom's Tractor, we have used the ownership as criterion for individualizing one of the tractors, the one that belongs to Tom.

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