Some animals that possess a language can refer to both concrete objects and abstract ideas. If a system of sounds or gestures contains no expressions referring to concrete objects or abstract ideas, then that system is not a language. If an animal's system of sounds or gestures is not a language, then that animal is unable to entertain abstract ideas. Animals do not have the cognitive capabilities to entertain abstract ideas.
Zoologist: Animals can certainly signal each other with sounds and gestures. However, this does not confirm the thesis that animals possess language, for it does not prove that animals possess the ability to use sounds or gestures to refer to concrete objects or abstract ideas.
Which one of the following is an assumption on which the zoologist's argument depends?