During the 18th century, mathematicians and philosophers were using the product "force times displacement" in order to calculate vis viva, whereas engineers used the same product in order to calculate moving engines' work and effectiveness. Physical magnitude "work", as a measure of labor, appeared in France at the beginning of 19th century, as a consequence of the synthesis produced when aspects of these two practices were combined. Only then, the expression "force times displacement" is regarded to construct the physical magnitude "work". Why? Surveying the evolution of relevant ideas in the history of science as well as views in the philosophy of science regarding the meaning of scientific concepts, I have formulated a number of criteria that determine when a mathematical expression, like "force times displacement", is considered autonomous physical magnitude, embedded in a theory.