In order to comprehend the ontological construction below, please refer to other my post for all notions in italic.
The law of supply and the law of demand cannot be severed from each other: decreasing – or respectively increasing – price for a commodity, by definition, entails increasing – or respectively decreasing – price of another commodity. When one of the goods is subject to the law of supply, then the other good is obeying the law of demand. And all is the same in reverse. Even in respect to a commodity taken on its own, the supply and the demand for it have the opposing tendencies prescribed by the law of supply and the law of demand simultaneously.
Historical Backdrop
• JOHN STUART MILL Principles of Political Economy: real law of supply and demand.