It is a tribute to the software designers, who have created a marvelous simulation of the real thing. The first time I played the game, I discovered something interesting. While all animals have the capacity to acquire knowledge, they must also know where, when and how to use it. Simply put, discrimination is the ability to decide that two events or things are unrelated.
Generalization is the ability to see a relationship between things.
So, to avoid cactus, a dog must able to discriminate between cactus and non-prickly objects and be able to generalize that all cactus should be avoided. In your home, these twin abilities are regularly displayed. The dog has decided that some aspect of the real doorbell is unique, and not shared with the TV bell. Another example of these dual behavioral principles occurs when you take your very obedient dog to a new location. The same dog who can sit, lie down, roll over, fetch and speak will become a distracted fool the instant he sets foot in a park.