Deductive Logic Informative Summary

Overview: This eBook is a comprehensive guide to deductive logic, written by St. George Stock in the late 19th century. It aims to reflect the logic teaching of the Oxford Schools at the time, heavily influenced by Aristotelian methods. The book systematically breaks down deductive reasoning into its fundamental components, starting with terms, moving onto propositions, and finally tackling inferences. The emphasis is on formal logic, focusing on the structure and validity of arguments rather than their content or truth in relation to the real world.

Key points covered include the division of terms into abstract/concrete, singular/common, positive/privative/negative, etc. The book delves into the quantity and quality of propositions, exploring different types like universal affirmative, particular negative, and their relationships in opposition (contrary, contradictory). A significant portion is dedicated to conversion (simple and per accidens), permutation, and their combinations for forming new inferences. The book then explores syllogisms, outlining the four figures, their valid moods, and the traditional canons of reasoning like the Dictum de Omni et Nullo. It details the methods of reduction, both direct and indirect, to demonstrate how arguments in imperfect figures can be translated to the first figure. The latter sections cover complex propositions (conjunctive, disjunctive) and syllogisms, including dilemmas. Finally, Stock provides a detailed analysis of common fallacies, both in language (equivocation, amphiboly) and in thought (accident, begging the question), drawing heavily from Aristotle’s work on sophistical refutations.

Key Findings:

  • Deductive logic is a system for determining the relative truth of propositions based on given premises, regardless of their actual truth in the real world.
  • All thought involves comparison, and the core of logical analysis involves classifying and comparing terms, propositions, and inferences based on their various characteristics.
  • A sound syllogism must adhere to strict rules regarding the distribution of terms, the quality and quantity of propositions, and the arrangement of terms in figures.
  • Many common arguments, though seemingly persuasive, rely on fallacies, either due to ambiguous language or flaws in reasoning.
  • Understanding the forms of valid argument and common fallacies equips us to both construct stronger arguments and critically evaluate those of others.


  • Nature of Deductive Logic: You’ll learn that deductive logic deals with the form of arguments, ensuring conclusions are consistent with given premises, without guaranteeing their material truth.
  • Classifications of Terms: The book provides a thorough grounding in different types of terms (e.g., abstract vs. concrete, singular vs. common, positive vs. negative), helping you understand their meaning and function in propositions.
  • Propositional Logic: You’ll gain a deep understanding of different propositional forms (A, E, I, O) and their relationships in terms of opposition, conversion, and permutation, allowing you to derive new inferences from existing propositions.
  • Syllogistic Reasoning: The book delves into the structure of syllogisms, teaching you to identify their figures, moods, and to evaluate their validity by applying specific rules.
  • Reduction Techniques: You’ll learn the methods of reducing syllogisms in imperfect figures to the first figure, demonstrating their underlying adherence to the Dictum de Omni et Nullo.
  • Complex Arguments: The book covers complex propositions (conjunctive, disjunctive) and their role in constructing more intricate arguments, including dilemmas.
  • Fallacy Identification: You’ll learn to identify common fallacies, both those arising from ambiguous language (equivocation, amphiboly) and those stemming from flaws in reasoning (accident, begging the question, etc.).

Historical Context: Stock’s book reflects the state of logical teaching in late 19th century Oxford, heavily grounded in the Aristotelian tradition. While influenced by thinkers like Mill and Hamilton, the book remains rooted in classical methods of analyzing the structure and forms of argument, with less emphasis on inductive reasoning or the practical applications of logic in everyday contexts.


  1. All thought involves comparison: This is the foundation of logic, as it focuses on recognizing similarities and differences between ideas.
  2. A term can be singular or common: Singular terms refer to one specific thing, while common terms refer to a class of things.
  3. Abstract terms denote attributes: Concrete terms denote substances.
  4. Propositions are judgments expressed in words: They compare two concepts or terms.
  5. The copula signifies agreement or disagreement: In its simplest form, it is represented by “is” or “is not.”
  6. Propositions can be universal or particular: Universal propositions apply to all members of the subject class, while particular propositions apply to some.
  7. Negative propositions distribute their predicate: Affirmative propositions do not.
  8. Contradictory propositions cannot both be true: They also cannot both be false.
  9. Contrary propositions cannot both be true: They can both be false.
  10. Conversion involves transposing subject and predicate: This can be simple or by limitation (per accidens).
  11. Permutation involves changing a proposition’s quality and replacing the predicate with its contradictory: This retains the meaning while altering the form.
  12. Syllogisms consist of three propositions: Two premises and a conclusion.
  13. Syllogisms have three terms: Major, minor, and middle.
  14. The middle term must be distributed at least once: Otherwise, there is no valid connection between the premises.
  15. Illicit process occurs when a term is distributed in the conclusion but not in the premise: This is an invalid form of argument.
  16. A conjunctive proposition asserts that the truth of the antecedent implies the truth of the consequent: For example, “If it rains, the ground is wet.”
  17. A disjunctive proposition asserts that at least one of the alternatives must be true: For example, “Either it is raining or it is not raining.”
  18. A dilemma is a complex argument with a conjunctive major premise and a disjunctive minor premise: It forces a choice between undesirable alternatives.
  19. Fallacies can be formal or material: Formal fallacies violate the rules of logic, while material fallacies rely on false or misleading premises.
  20. Equivocation occurs when a term is used with different meanings in the same argument: This can lead to invalid conclusions.


  1. Term: A word or group of words that can serve as the subject or predicate of a proposition.
  2. Proposition: A judgment expressed in words, consisting of a subject, copula, and predicate.
  3. Syllogism: A type of deductive argument consisting of two premises and a conclusion, where the conclusion follows logically from the premises.
  4. Distribution: A term is distributed when it refers to all members of the class it denotes.
  5. Conversion: An immediate inference that involves transposing the subject and predicate of a proposition.
  6. Permutation: An immediate inference that involves changing the quality of a proposition and replacing the predicate with its contradictory.
  7. Conjunctive Proposition: A proposition that asserts the truth of the antecedent implies the truth of the consequent (e.g., “If A, then B”).
  8. Disjunctive Proposition: A proposition that asserts at least one of the alternatives must be true (e.g., “Either A or B”).
  9. Dilemma: A complex syllogism with a conjunctive major premise and a disjunctive minor premise, forcing a choice between undesirable alternatives.
  10. Fallacy: An error in reasoning that results in an invalid argument.


  1. “All men are mortal”: This is a universal affirmative proposition (A), where both the subject (“men”) and the predicate (“mortal”) are distributed.
  2. “Socrates is a man. All men are mortal. Therefore, Socrates is mortal”: This is a classic example of a syllogism in the first figure, mood Barbara.
  3. “No fish are mammals. All whales are mammals. Therefore, no whales are fish”: This is a syllogism in the second figure, mood Cesare.
  4. “If it is raining, then the ground is wet. The ground is wet. Therefore, it is raining”: This is an example of the fallacy of affirming the consequent.
  5. “All dogs are mammals. All mammals have four legs. Therefore, all dogs have four legs”: This is a valid sorites argument.
  6. “The Bible says God exists. The Bible is the word of God. Therefore, God exists”: This is an example of the fallacy of begging the question.
  7. “Light is a wave. A wave is a movement. Therefore, light is a movement”: This is an example of the fallacy of equivocation.
  8. “You should support this policy because it is the patriotic thing to do”: This is an example of the argumentum ad populum.
  9. “This man is a criminal, therefore his argument is wrong”: This is an example of the argumentum ad hominem.
  10. “Have you stopped beating your wife?”: This is an example of the fallacy of many questions.

Conclusion: This ebook provides a detailed and systematic exploration of deductive logic, outlining its principles, methods, and common pitfalls. By mastering the concepts and techniques presented, readers can enhance their ability to construct sound arguments, identify fallacies in reasoning, and engage in more rigorous critical thinking. The book underscores the importance of precision in language and the power of formal logic in ensuring consistency in thought, even if it does not guarantee the material truth of conclusions.

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