Logic Charts

Logic and Inquiry: Trent Dougherty

Trent.Dougherty @ Rochester.edu

Types of Proposition

Quantity: Universal or Particular

Quality: Affirmative or Negative

N.B. “All S is not P” = “No S is P”

Distribution:

A: DU

E: DD

I: UU

O: UD

Rules of Syllogism: (the rules are IA, IB, IIA, etc.)

I. Formation Rules: The Rules of Three

  A. Exactly Three Propositions

    1. Two (2) Premises

    2. One (1) Conclusion

  B. Exactly three terms

    1. One Major Term: The Predicate Term of the Conclusion

    2. One Minor Term: The Subject Term of the Conclusion

    3. One Middle Term: The "Bridge" Term which Appears Twice, Once in Each Premise

II. Weakest Link Rules

  A. Quantity: If one premise is particular, then the conclusion must be particular.

  B. Quality: If one premise is negative, then the conclusion must be negative.  .

  C. No conclusion at all from two negative premises.  "Nothing comes from nothing."

III. Distribution Rules (can also be considered weakest link rules, ask me)

  A. Middle term must be distributed at least once.

  B. Any term distributed in the conclusion must be distributed in the premise.

If a syllogism follows all these rules--if you build an argument according to this recipe--then it is VALID.  That is, IF the premises are true, THEN the conclusion *must* be true as well. 

Validity by Figure and Mood

Figure: Shape made by arrangement of middle term.

Mood: Triplet of Proposition-types.