“Since the seventeenth century it has been customary to treat so-called virtus dormativa explanations with scorn. A virtus dormativa explanation gets its name from Molière’s play Le Malade imaginaire, in which a foolish doctor is asked how a certain powder induces sleep. He replies that it has a virtus dormativa — a power of inducing sleep. The heart of the objection to virtus dormativa explanations is that they are not explanations at all. To say that a powder causes sleep because it has a power to bring on sleep is to explain nothing: it is just to repeat that the powder causes sleep.
“There is no doubt but that Molière’s doctor is a fool and his ‘explanation’ is a sham. But his legacy to Western culture is, I believe, a mistaken conception of an adequate explanation. It is widely believed that if any explanation has the structure of a virtus dormativa explanation, it must therefore be circular and non-explanatory. Thus, Aristotelian powers are viewed as inevitably suspect. This, I think, is a mistake. There may be a valid objection to certain explanations which have a virtus dormativa structure, but the objection is not one of principle. Even if we do not live in Aristotle’s world, it is not absurd to imagine that our world is as he described. In such a world we could not explain the organism’s ability to develop in terms of its material microstructure. In such a world this ability is form, and form is one of the basic constituents of the universe: it cannot be explained in terms of anything more fundamental.
“In Aristotle’s world form as a potentiality or power does help to explain the growth, development and mature functioning of living organisms. And there are empirical tests for the presence of form. Were there no structure in an immature organism or regularity in the process of development there would, in Aristotle’s eyes, be no basis for the attribution of a power, regardless of the outcome. The absurdity of Molière’s doctor is manifested not merely by his virtus dormativa explanation but, first, by the fact that he has not noticed that he is not living in Aristotle’s world (and by that time in the history of science he should have); second, by the evident fact that he merely cites the virtus without having any understanding of how it might work as an explanation; third, by the fact that he has done nothing to determine whether the powder actually has the power. (He could have devised tests to distinguish accidental onset of sleep from genuine inducement.)”
Johnathan Lear, “Aristotle: the Desire to Understand” (Cambridge University Press, 1988), pp. 23-24.