"Dormitive1 virtue" (or "virtus dormitiva") is used in the philosophy of science to refer to the classic fallacy of defining an effect as its own cause. It is based on a line from the 1673 Moliére play Le Malade Imaginaire (The Hypochondriac), wherein a doctor explains that opium is a soporific "quiat est in eo / virtus dormitiva / cujus est natura / sensus assoupire" ("because there is a dormitive virtue in it whose nature is to cause the senses to become drowsy"). Explanations along these lines answer questions truthfully but vacuously. They state that there exists something that causes the effect in question, but give no insight into what that might be.

A number of pseudoscientific concepts take part in the dormitive virtue fallacy. Vitalism, for example, resolves the question of "what causes or constitutes life?" by positing a "life force" -- essentially, living things are alive because they have some sort of "vital virtue." But even solid-seeming theories can boil down to a postulation of dormitive virtue. For instance, a number of theories in the philosophy of mind end up describing consciousness as the brain's capacity for awareness -- a "conscious virtue." The circularity of these theories is not always inherently obvious.

"Dormitive virtue" has a long history as a philosophical concept. David Hume refers to it in his Dialogues, and Friedrich Nietzsche uses it in Beyond Good and Evil to deride Immanuel Kant's claim that humans have a faculty which allows them to gain a priori synthetic knowledge. Charles Peirce, by contrast, defends Moliére's doctor; he argues that the distinction between the concrete concept "puts people to sleep" and the abstract concept "has a dormitive virtue" is actually pragmatically significant. There's no semantic difference between the statements (they mean the same thing) but there is a pragmatic difference (they have different effects on the interpreter). He classified "opium puts people to sleep" as a first-order predicate, and "opium possesses a dormitive virtue" as second-order.

Some might argue that Peirce is being a spoilsport here, as usual,2 and he is at least being typically iconoclastic. For the most part, it's assumed that something partaking of the "dormitive virtue" fallacy cannot possibly lead to fruitful inquiry. However, it should be noted that there are instances in which the dormitive virtue explanation actually does some work. If one asks why opium puts people to sleep, then the answer "because it is a soporific" is vacuous. But if one asks "why did I go to sleep after taking opium," then the same answer is actually somewhat explanatory. It begs certain questions3, yes, but it is not wholly circular. In these cases, however, it is still necessary to then examine these properties of dormitiveness if one is going to make any scientific progress.

1 It appears that this can also be spelled "dormative."
2 I like Peirce, and if I'm down with any -ism, it's pragmatism, for sure. But I still think pragmatists can be a real pain in the butt to argue with.
3 I used this wrong.

because it takes me a while to understand, more than because I'm a good researcher:
http://www.artsci.wustl.edu/~philos/MindDict/knowhow.html
assets.cambridge.org/0521770785/ sample/0521770785WS.PDF
http://www.dunbar-i-l.demon.co.uk/philosophy/scisen_5.htm
http://mars.virtual-earth.de/pipermail/cg/1991q4/001385.html
http://sow.colloquium.co.uk/~barrett/preface.html
http://www.clas.ufl.edu/users/jzeman/csphiloflogic.htm

thank you JerboaKolinowski

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