To prevent his system from being seen as mechanistic or reductionist while, also trying to draw a basic distinction between animal and human learning, Pavlov wrote in 1927: "Of course a word is for man as much a real conditioned stimulus as are other stimuli common to men and animals, yet at the same time it is so all-comprehending that it allows no quantitative or qualitative comparisons with conditioned stimuli in animals." In 1932, Pavlov made the claim that speech is signals of signals—or second signals. These second signals are, at their core, abstractions of reality and a method of generalization that is distinctive of human higher thought. He stated, again in 1932: "In man there comes to be .. . another system of signalization, a signalization of the first system . . . a new principle of neural action is introduced". This is the Pavlov "second-signal system" principle that distinguishes verbal conditioning, or language acquisition, in man from first-signal conditioning in men and animals. This view is different from most American behaviorists, who claim language is either a mediator, operating principally according to the laws of the reactions that it mediates, or is simply a conditioned vocal reaction.