Publication 936 says you can deduct mortgage interest on a home other than main/second home if loan proceeds used for business, investment, or "other deductible purposes".  But there's no further discussion on that last condition.  Does anyone have know how/where that phrase is defined?

My example may distract from my question... Parents refinance house to pay off kid's student loans - kid now pays mortgage on parent's house - can kid deduct mortgage interest as student loan interest or mortgage interest or neither?