# What are ontologies in AI?

What exactly are ontologies in AI? How should I write them and why are they important?

Ontologies are similar to taxonomies. A taxonomy is a tree-like hierarchy that organizes concepts in increasing levels of specificity. What an ontology adds is a second type of link between those concepts that explains how they are connected. So a taxonomy could say $$isA(DOG, ANIMAL)$$ ("a dog is an animal"). An ontology may also describe something like: $$chase(DOG, CAT)$$ ("dogs chase cats").