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?