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This lesson provides a detailed examination of the logical fallacies that appear most frequently in UCAT Decision Making syllogism questions. Understanding why these errors are tempting — and why they are logically invalid — is the key to avoiding them under time pressure. Each fallacy is explained with its formal structure, a concrete example, and a Venn diagram analysis showing exactly where the reasoning breaks down.
The UCAT tests whether you can distinguish valid from invalid reasoning. The test designers deliberately construct answer options (and proposed conclusions in Yes/No items) that commit well-known logical fallacies. These fallacies are chosen because they are psychologically persuasive — they feel right even though they are logically wrong.
Your defence is to learn the fallacy patterns so thoroughly that you recognise them instantly.
| Premise 1 | All A are C |
|---|---|
| Premise 2 | All B are C |
| Fallacious conclusion | Therefore, some A are B |
Both A and B are subsets of C, but they could be entirely separate subsets. Imagine C as a large box. A is a smaller box inside it on the left; B is a smaller box inside it on the right. They share no members.
All cardiologists are doctors. All dermatologists are doctors. Therefore, some cardiologists are dermatologists.
This is invalid. The set of cardiologists and the set of dermatologists are both inside the set of doctors, but they do not overlap at all. No cardiologist is a dermatologist.
Look for two premises that both connect different subjects to the same predicate using "All...are." If the conclusion then claims a connection between the two subjects, it is almost certainly committing the Undistributed Middle fallacy.
| Premise 1 | If P, then Q |
|---|---|
| Premise 2 | Q is true |
| Fallacious conclusion | Therefore, P is true |
Equivalently, using categorical logic:
| Premise 1 | All A are B |
|---|---|
| Premise 2 | X is B |
| Fallacious conclusion | Therefore, X is A |
"All A are B" tells us that being A is sufficient for being B. It does NOT tell us that being B is sufficient for being A. There may be other ways to be B without being A.
If a patient has meningitis, they will have a headache. This patient has a headache. Therefore, this patient has meningitis.
This is invalid. Headaches have many causes — migraine, tension, dehydration, etc. Having a headache does not prove meningitis.
Look for a conclusion that reverses the direction of an "if...then" or "all...are" statement. If the premise says "A leads to B" and the conclusion says "B therefore means A," it is Affirming the Consequent.
| Premise 1 | If P, then Q |
|---|---|
| Premise 2 | P is false (not P) |
| Fallacious conclusion | Therefore, Q is false (not Q) |
The conditional "If P, then Q" tells us what happens when P is true. It says nothing about what happens when P is false. Q might still be true for other reasons.
If a student passes all their exams, they will graduate. James did not pass all his exams. Therefore, James will not graduate.
This is invalid. Perhaps James can resit exams, or perhaps there is a compensatory mechanism. The premise only tells us what passing all exams guarantees; it does not tell us that failing any exam prevents graduation.
Look for a conclusion that says "not Q" based on the information "not P," where the premise links P to Q. The premise guarantees Q when P is true — but says nothing about when P is false.
| Premise 1 | All A are B |
|---|---|
| Premise 2 | No C are A |
| Fallacious conclusion | Therefore, no C are B |
The conclusion makes a claim about ALL of B (that no C are in B), but Premise 1 only tells us about A's relationship to B. There could be members of B outside A, and some of those could be C.
All psychiatrists are doctors. No nurses are psychiatrists. Therefore, no nurses are doctors.
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