2.4.1 Abstraction Hierarchies
New concepts can be created by abstracting from existing ones. This means disregarding certain aspects of a concept in favour of others. Starting with the concept dog, we can disregard certain properties of dogs and arrive at the more abstract concept mammal, which in turn can be abstracted to animal, etc. Abstraction is often useful when given a large number of concepts with the aim of finding out what they have in common. To achieve this a new, more general concept than the previously given ones is constructed. For example, the concepts triangle, square, rectangle, pentagon, and hexagon can be abstracted to polygon. The more general concept is called a generalisation of the more specific one. Conversely, the more specific concept is called a specialisation of the more general one. So, mammal is a generalisation of dog and a specialisation of animal. Note that every property that can be attributed to a concept can also be attributed to a specialisation of it, but the opposite does not hold true. For instance, the properties weight, colour, and brain volume, which are relevant to mammals are also relevant to dogs, but the property tail length which is relevant to dogs is not generally applicable to mammals.