Only human beings have the power of intellectual abstraction; therefore, only human beings can form a general or universal concept. Irrational animals have the external and internal senses, which are sometimes keener than those of humans. But because they lack the rational powers (intellect, intellectual memory, and free will), they are incapable of progress or of culture. Despite their remarkable instinct, their productions, intricate though they may be, remain the same through the centuries, for example: beaver dams, bird nests, anthills, beehives.