Two Irishmen purchase horses from a farmer

As they ride away, one says to the other "Paddy, how are we to tell our two horses apart?".
"Well, Seamus, 'tis simple: I'll cut my horse's ear, and that will show us it's my horse!"...and he cuts his horse's ear.
Ten minutes down the road, they run into some brambles, and Seamus' horse's ear gets an identical cut in its ear.
This causes the same argument to come up again, until Paddy says "Seamus, I'll cut my horse's tail off, and that will show us it's my horse!"
Seamus finds this acceptable, until, ten minutes later, they encounter MORE brambles, which rip off Seamus' horse's tail just like Paddy's.
They ride and ponder the problem for a mile or so, until Paddy suddenly proclaims "I've got it Seamus! You keep using the black horse, and I'll keep using the brown one!"