Other objects are vague too... Rowing boats are repaired by replacing planks. When no plank is the same, is it the same boat? Think of a book or PhD thesis. How is it written? Doesnt it change all the time? What if its translated? What if its translated badly? How many languages are spoken in China? Every linguists gives a different answer. Are you the same person as 3 weeks after conception? After youve lost your memory? The cat is loosing hair. Is it still part of the cat?