Yet, this ex juvantibus line of reasoning (i.e., reasoning “backwards” to make assumptions about disease causation based on the response of the disease to a treatment) is logically problematic—the fact that aspirin cures headaches does not prove that headaches are due to low levels of aspirin in the brain.

I came across this phrase in "The 21st-Century Brain" by Steven Rose (a very interesting book!) The phrase yields about 16,000 Google-hits. It is, I gather, a term for a kind of "non causa pro causa" error in diagnostics.

What is the origin of this phrase? Who's Juvantibus when he's at home?

My guess would be it's a Dark Ages/vulgar Latin corruption of "ex iuventibus" - iuventibus would be the plural ablative form of iuventus (3rd decl. fem.) - literally, from youths, from young men, or from the prime of life.

No need to assume corruption. Latin juvo 'to help, assist, support, benefit; please'; juvans is simply the present participle 'helping'. A common phrase was dis juvantibus or deo juvante, meaning 'with god's help'.

Yes, "from that which helps" is much better. However, there's still a bit "corruption" because there was no 'J' in Latin till the Middle Ages. If you use a classical Latin dictionary, you still need to look the word up under 'I'.

If you use a classical Latin dictionary, you still need to look the word up under 'I'.

It depends on the dictionary you use. It's really a stylistic convention. I looked ivvo up in Lewis and Short, which is about as classical as the dictionaries in my library get. It's under J for consonantal I. You could argue that a Roman Latin dictionary would list ivvo under IVVO as the Romans mainly used capitals letters, but then you wouldn't find a Latin-English dictionary in Ancient Rome.

[Edited typo.]

I only JUST discovered this phrase in an 1989 Anesthesia paper on a certain syndrome - that the diagnosis was made "ex iuvantibus" by admisitering a certain drug.
It is certainly something we do a lot, but never heard the term before...
Rm

Syndrome ex iuvantibus--Very large upright barrel accommodating eager standup lovemaking by adolescents intent on demonstrating their adulthood. Set in rotation on its axis at at high speed, whereupon bottom drops out, centrifugal force holding occupants against wall

Some folks call these zombie threads.

That explains where the brains went.

and I thought Rainmaker was back.

:¬ (