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http://dx.doi.org/10.1037/13326-014
In our recital of differences between man and the other animals, we must remember that man has the capacity to acquire language and that animals do not. If it were a matter of opportunity only, many pet dogs and cats would acquire a better language than some children. It is true that some animals, notably the parrot, may learn to say several words; but saying words is not using language. Language consists in using certain sounds, movements, marks, or other kinds of sensory stimuli as signs for certain things. These signs must be understood by both the producer and the receiver of the sign. (PsycINFO Database Record (c) 2016 APA, all rights reserved)