 The Islamic condemnation of interest, or riba, adds an additional, third line of argument which holds that interest is essentially unearned or undeserved income: Since the lender neither partakes in the actual productive use of the money lent, nor exposes him- or herself to commercial risk, the lender cannot legitimately share in the gains produced by the loan (Ayub 2007, Birnie 1952, Thomas 2006). Based on this argument, contemporary Islamic banks insist that lenders and borrowers must form a business partnership in order for fees on loans to be morally legitimate (Ayub 2007, Warde 2010). Economists have over the years given several retorts to this argument. Some economists stress that lending also involves risk (e.g., that the borrower defaults and is unable to repay); others stress the so-called opportunity costs of lending (i.e., that the money could have been used more profitably elsewhere); and yet again others stress the simple time-preference of individuals (i.e., that we value present more than future consumption, and therefore the lender deserves compensation for postponing consumption).