the modal fallacy

ah, the english language; a source of wonderful beauty and mind numbing consternation. the way we naturally express our thoughts in english doesn't help when trying to explain very specific philosophical nuances. the modal fallacy goes like this. we would naturally say something like this:

if john has one son and two daughters, he has to have at least one daughter.
john has one son and two daughters.
therefore john has to have at least one daughter.

but think about it... that's rubbish. he doesn't' have to have any kids; daughters or sons. it is true that if john has one son and two daughters he will have at least one daughter, but it's not true that he must. in other words it is incorrect to mistake a logically necessary condition, as a logically necessary truth.

this holds for a number of philosophical problems. they are dependent on this misusing of modal concepts. if you're interested, please check out this article by Norman Swartz.