excerpt from: http://www.psychologytoday.com/blog/canine-corner/201107/what-are-the-limits-canine-learning

"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"