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.
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.— H. W. B. Joseph
All persons are mortal.
Socrates is a person.
Therefore, Socrates is mortal.
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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]
- Walton, Douglas (1990), "Ignoring Qualifications (Secundum Quid) as a Subfallacy of Hasty Generalization" (PDF), Logique et Analyse 33 (129-130): 113–154, ISSN 2295-5836
- Parry, William T.; Hacker, Edward A. (1991), "A brief history of the fallacies of accident and secundum quid", Aristotelian Logic, SUNY Press, p. 438, ISBN 978-0-7914-0690-8
- Bunnin, Nicholas; Yu, Jiyuan (2008), "Fallacy of secundum quid", The Blackwell Dictionary of Western Philosophy, John Wiley & Sons, p. 250, ISBN 978-0-470-99721-5
- Joseph, H. W. B. (1916), "The fallacy of Secundum Quid", An Introduction to Logic (2nd ed.), Clarendon Press, p. 589, OCLC 373124
- Coffey, Peter (1912), The Science of Logic, Longmans, Green, and Company, p. 309, OL 7104938M