The great discovery in this case was the notion of interesse, which is where our word “interest” originally comes from: a compensation for loss suffered because of late payment.124 The argument soon became that if a merchant made a commercial loan even for some minimal period (say, a month), it was not usurious for him to charge a percentage for each month afterward, since this was a penalty, not rental for the money, and it was justified as compensation for the profit he would have made, had he placed it in some profitable investment, as any merchant would ordinarily be expected to do.125
