I saw this mentioned in passing on another web site and realized that it's basically what I did with a terrier I once had many moons ago. Back then I was working 3rd shift and was working multiple 12 hour shifts in a row. I did not have time during the day to let a dog outside. When you've just worked 3 12's in a row and you've got 8 hours before 12 hour shift #4 you want to spend as much of that 8 hours sleeping as you possibly can. It was 20 degrees outside so I couldn't put the dog outside and leave him as that would be cruel by any definition. I put the dog in his crate, took the crate door off and pushed it up to the dog door which I left unlatched. When the dog had to go, he wouldn't want to go in his crate so he would walk out the dog door and do his business out there. Essentially, he kind of house trained himself as he developed the habit of heading out the dog door when he needed to pee.