Closed versus Open World Assumption

Let us take an example and define the following tripels:

• dog is_a mammal;
• cat is_a mammal;
• mammal is_an animal;
• fish is_an animal
• mammals have vertebra;

Based on this knowledge do have fishes vertebra ?
A first interpretation is to say that fishes have no vertebra because it is not explicitly specify. This is the "Closed World Assumption". The second interpretation says that the non-specification means that we do not know. This is the "Open World Assumption".