Final answer:
The demand for labor is derived from the demand for the firm's output.
Step-by-step explanation:
Economists describe the demand for inputs like labor as a derived demand. Since the demand for labor is MPL*P, it is dependent on the demand for the product the firm is producing. An increase in demand for the firm's product drives up the product's price, which increases the firm's demand for labor. Thus, we derive the demand for labor from the demand for the firm's output.