<Ul> <Li> Undistributed middle: Neither of the premises accounts for all members of the middle term, which consequently fails to link the major and minor term . </Li> <Li> Illicit treatment of the major term: The conclusion implicates all members of the major term (P--meaning the proposition is negative); however, the major premise does not account for them all (i.e., P is either an affirmative predicate or a particular subject there). </Li> <Li> Illicit treatment of the minor term: Same as above, but for the minor term (S--meaning the proposition is universal) and minor premise (where S is either a particular subject or an affirmative predicate). </Li> <Li> Exclusive premises: Both premises are negative, meaning no link is established between the major and minor terms . </Li> <Li> Affirmative conclusion from a negative premise: If either premise is negative, the conclusion must also be . </Li> <Li> Negative conclusion from affirmative premises: If both premises are affirmative, the conclusion must also be . </Li> <Li> Existential fallacy: This is a more controversial one . If both premises are universal, i.e. "All" or "No" statements, one school of thought says they do not imply the existence of any members of the terms . In this case, the conclusion cannot be existential; i.e. beginning with "Some". Another school of thought says that affirmative statements (universal or particular) do imply the subject's existence, but negatives do not . A third school of thought says that the any type of proposition may or may not involve the subject's existence, and though this may condition the conclusion, it does not affect the form of the syllogism . </Li> </Ul> <Li> Undistributed middle: Neither of the premises accounts for all members of the middle term, which consequently fails to link the major and minor term . </Li> <Li> Illicit treatment of the major term: The conclusion implicates all members of the major term (P--meaning the proposition is negative); however, the major premise does not account for them all (i.e., P is either an affirmative predicate or a particular subject there). </Li> <Li> Illicit treatment of the minor term: Same as above, but for the minor term (S--meaning the proposition is universal) and minor premise (where S is either a particular subject or an affirmative predicate). </Li>

Some a are not b. (t) no a are b