It is difficult to know exactly how much a dog can learn, and recent data keep pushing the limits beyond what we felt was possible before. Perhaps one breakthrough, in terms of our ability to assess the intelligence of dogs came about in the early 1990s. At that time it dawned upon me that one way to learn about the limits of canine abilities was to use tests that were already developed for assessing human infants, and to modify them so that they could be used for dogs. The idea was that if a dog could pass a particular test, then not only would he have clearly demonstrated that he has the fact particular mental ability, but it might be possible to assign a human mental age to his performance, which might give us a better understanding of the dog's mental capacity. A number of canine behavioral researchers ultimately adopted the same strategy.
If we are to draw a conclusion from this kind of research, it is that dogs have the mental ability approximating that of humans between two and three years of age, at least as far as language, object recognition, and concept formation. That means that if we are posing a problem, or teaching a task, which would be too difficult for a human two or three-year-old to solve or learn, then it is likely to be beyond the dog's capacity as well.