In general it was censurable, usury, to charge interest on sums that were lent, because money was deemed unproductive; however, it was morally acceptable to receive compensation, a return or a share of the profits where the money lent gave rise, through commerce, to the creation of new wealth.
The distinction between monetary interest and profits, the fruit of the use of money in commercial and productive enterprises, was lost for several centuries. After Wicksell it was rediscovered by Keynes, who made it a cornerstone of his General Theory.