Is it Fair to Leave a Dog Alone All Day?

Sorry but i think if you are going to be a responsible loving pet owner you shouldnt have a dog if you are going to be at work fulltime. That is a recipe for disaster and would be like leaving a child at home alone. Different if the dog is older.

However, if you have other animals in the house, it makes a difference; and the during-the-day visits are always good.
   
I also have 2 cats - we adopted them a few months after we adopted Marlowe - so he actually isn't alone all day, there are other warm bodies in the house.
   
He's decided that the younger cat, Faro (a tuxedo cat who looks like he stepped off of an Egyptian tomb painting) is a recalcitrant puppy that he has to supervise, so he actually has in interest during the day. (Marlowe is a natural baby-sitter.) (Tuxedo cats are half dog, anyway...) And Faro doesn't seem to mind the supervision thing, so it works out for all involved.
   
And Marlowe's certainly better off having a permanent home than still being in a foster home. He needed one-on-one attention, lots of love, & the chance to get over an undiscovered (until we got him) fear of men. We're Home #4 in his life (he was 2 1/2 when we got him), so he's got stability now.
   
Even a working pet owner is better than the dog being in a foster home or a shelter.
   
Dogs adapt better to being alone for long periods of time than parrots.

I keep seeing posts that say a dog does not have a concept of time. Does anyone know of a dog behavior test that taught this to the experts and why it was determined? I know if I walk out for 15 minutes, my dogs don't do much...but if I am gone all day they jump and bark (greetings) like crazy as though they are very happy to see me. So, how do we know they have absolutely no concept of time? I would be interested to know how that was determined. just wondering.