Secundum quid

Secundum quid (also called secundum quid et simpliciter, meaning "[what is true] in a certain respect and [what is true] absolutely") is a type of informal fallacy that occurs when the arguer fails to recognize the difference between rules of thumb (soft generalizations, heuristics that hold true as a general rule but leave room for exceptions) and categorical propositions, rules that hold true universally.

Since it ignores the limits, or qualifications, of rules of thumb, this fallacy is also named ignoring qualifications or sweeping generalizations. The expression misuse of a principle can be used as well.

Example
"Let me tell you: all great composers die young. Take Mendelssohn: he was 38. Or Mozart, just 35. And Schubert! Hundreds of songs, and he was only 31."

The arguer cites only the cases that support his point, conveniently omitting Bach, Beethoven, Brahms etc.

"Water boils at a temperature of 212 °Fahrenheit; therefore boiling water will be hot enough to cook an egg hard in five minutes: but if we argue thus at an altitude of 5,000 feet, we shall be disappointed; for the height, through the difference in the pressure of the air, qualifies the truth of our general principle."

Compare with: "All men are mortal. Socrates is a man. Therefore, Socrates is mortal."

In popular culture
The following quatrain can be attributed to C. H. Talbot:

I talked in terms whose sense was hid, Dividendo, componendo et secundum quid; Now secundum quid is a wise remark And it earned my reputation as a learned clerk.

Types
Instances of secundum quid are of two kinds:
 * Accident — a dicto simpliciter ad dictum secundum quid (where an acceptable exception is ignored) [from general to qualified]
 * Converse accident — a dicto secundum quid ad dictum simpliciter (where an acceptable exception is eliminated or simplified) [from qualified to general]