> When I walk into a room containing my dog and the poo it did on the carpet two hours earlier, it is sorry about what it did.
This shows that the dog knows it did something you didn't want it to do, yes. I'm not sure it shows "thinking about ourselves, scrutinising and evaluating what we do and why we do it", which is how the article this discussion is about described "mental reflection". Our dogs have made messes in our house and have shown evidence that they know they're not supposed to, but I see no evidence that they have done any reflection on why they did it.
Of course "mental reflection" is not an all or nothing thing, obviously there is a continuum of possibilities. So a more precise phrasing of my question might be: is there any evidence that dogs can perform mental reflection at a point on that continuum anywhere near the point where humans do it? Or are they only capable of it at a point on the continuum much, much closer to the other end, the "no reflection at all" end?
I'm not sure since we don't have any good way of quantifying "amount of mental reflection". But that doesn't mean there isn't a large difference between dogs and humans in this regard.
