Let's say you think a property, like 'purpose', is a two parameter function and someone else tells you it's a three parameter function. An interesting thing to do is to accept that it is a three parameter function and then ask yourself which of the following holds:
1) The third parameter is useless and however it varies, the output doesn't change.
2) There is a special input you've been assuming is the 'correct' input, which allowed you to treat the function as if it were a two parameter function.