interest as damages for non-performance

It is a dictate of natural justice, and the law of every civilized country, that a man is bound in equity, not only to perform his engagements, but also to repair all the damages that accrue naturally from their breach. Hence, every nation, whether governed by the civil or common law, has established a certain common measure of reparation for the detention of money not paid according to contract, which is usually calculated at a certain and legal rate of interest. Every one who contracts to pay money on a certain day knows, that, if he fails to fulfil his contract, he must pay the established rate of interest as damages for his non-performance.