A person's observations of her dogs' behaviour
Besides scientific studies, there is a lot of anecdotal evidence on dogs' intelligence. Much of it, not surprisingly, is from people who have a dog. Some may argue that they are biased in favour of their dogs, but it's undeniable that these are the humans who best know dogs.
This is a piece by Joan Biggin from the April 2010 issue of Mensa Magazine, the publication for Mensa Society members, who should know a thing or two about intelligence:
I think dogs can learn and think. A Jack Russell we had some years ago used to look for things he saw on the TV. He tried sniffing the screen, then looked behind the set for a bit, ran out of the room and down the hall to the point where the set was in the room, but the other side of the wall.
We decided he must have connected the screen with a window, which would have the same perception of pictures without smell.
We had a lurcher who the local poachers thought would make a good hare dog. As he was a family pet and we discouraged him hunting, we thought this amusing.
So at walk time we would say 'Coming to find a hare, Sam?' It quickly became obvious that 'Go and brush your hair', said to either of our daughters, had the same effect on him.
'Hair' - he wanted a walk. It could not have been our tone of voice because this would be different in the two contexts; it would also be in different family situations and times of day.
He had simply learned the word. By the time he died, he had a large vocabulary of words he understood besides the usual commands.