Animals, too, communicate. If you try to catch a seagull, it will call out "Hahaha! Hahaha!" to signal its friends to watch out for an intruder (入侵者). Certain fish signal their presence by sending electric impulses from their muscles. But these signal systems differ in very fundamental ways from human communication.
First of all, our symbols are subjective. If you do not speak Chinese, you would not know what a gou is. Gou is the Chinese word for dog. There is no inherent connection between the word and the thing itself. The Spaniards, after all, call the same animal perro and the French call it chien. The meaning of a word is not determined by any inherent quality of the thing itself. It is instead subjective: a word may mean whatever a group of humans have agreed it is supposed to mean. On the other hand, animals are not free to produce different symbols subjectively to indicate the same thing. This is why, for example, all seagulls throughout the world make the same sound to indicate the presence of er.
Second, animal communication is a closed system, whereas human language is an open system. Each animal species can communicate only a limited set of messages, and the meaning of these signals is fixed. Animals can use only one signal at a time—they cannot combine two or more to produce a new and more complex message. A bird can signal "worms" to other birds but not "worms" and "cats" together. Animal communication is also closed in the sense of being tied to what is immediately present in the environment. The bird can signal "worms" only because it sees them. In contrast, we call blend and combine symbols to express whatever ideas come into our heads. We can create new messages, and the potential number of messages that we can send is infinite. Thus we can talk about concepts such as good and evil, truth and beauty, for which there is no physical thing that is being signaled. It is this creative character of language that leads many people to believe that language is unique to humans. Although several chimpanzees (黑猩猩) have been taught sign language, it is doubtful that they have created innovative sentences of their own. The main idea of Paragraph 2 is that ______.