F012 • Formal - Syllogistic Logic
Also known as: Illicit Process of the Major Term
We're making a universal claim about `B` based only on information about a part of `B`.
We learn something about a part of a category and find ourselves drawing a conclusion about the entire category, as though knowing about a slice of the pie told us about the whole thing. The conclusion stretches further than the premises ever reached.
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Pattern: All A are B; no A are C; therefore no C are B.
Terms:
A = Known subset (cats)B = Major category (mammals)C = Excluded group (dogs)Steps:
A belong to BA and C are separateC cannot belong to B eitherWhen we know something about a group, there is a natural momentum that carries us toward broader claims. If all cats are mammals, and no cats are dogs, it can feel like we have learned something about all mammals -- but we have only learned about the cats-and-mammals corner of the picture. The useful heuristic here is extrapolation: taking what we know about a part and extending it to the whole. That works surprisingly well in daily life, which is why the failure mode is hard to spot. The problem is that our premises made a claim about some mammals (the ones that are cats) and the conclusion quietly upgrades that to a claim about all mammals. The major term -- the big category in the conclusion -- gets distributed more broadly than the premises ever warranted. It is like surveying one neighborhood and drawing conclusions about an entire city.
| Extrapolation from parts to wholes is one of our most productive reasoning habits. It becomes valid when the premises genuinely do address the full scope of the category the conclusion claims to cover |
| When the major term is properly distributed in the major premise -- meaning the premise really does make a claim about all members of that category -- the conclusion is on solid ground |
| Watch for conclusions that make sweeping claims about a category when the premises only discussed part of that category |
| You might feel a conclusion is obvious, but when you look closely, the premises were talking about a subset while the conclusion is talking about everything. That gap is the illicit move |
| Ask yourself: did the premises tell me about all members of this group, or just some of them? If just some, the conclusion cannot safely claim more |
| Notice when a word appears in both a premise and the conclusion but seems to have quietly grown in scope -- covering more territory in the conclusion than it did in the premise |
| Struggling to identify which term has been improperly expanded, especially when the language feels natural and the conclusion seems intuitive |
| Confusing the major term (the predicate of the conclusion) with the minor term, which leads to misdiagnosing the error |
| Illicit Major |
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| The major term is distributed in the conclusion but not distributed in the major premise. |
| You cannot make a claim about all members of a category in the conclusion if you haven't made such a claim in the premises. |
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