The distinction is best appreciated if we use [] syntax
if ([door isOpen])
doSomething;
else
[door setOpen:YES];
reads more like plain English than
if ([door isOpen])
doSomething;
else
[door setIsOpen:YES];
in modern-day dot syntax the difference is a little lost
if (door.isOpen)
doSomething;
else
door.open = YES;
vs
if (door.isOpen)
doSomething;
else
door.isOpen = YES;