[Lecture Seven] Introduction to Logic
by Dr. Leonard Peikoff
Total Time: 2 hours, 45 minutes
Course summary: This lecture course by Dr. Leonard Peikoff provides a comprehensive introduction and overview of the study of logic. Through exercises provided to the reader and discussion of answers, the course covers definitions, syllogisms, fallacies, and the rules of generalization. It is equivalent to a university level course in logic. Read more »
In this lecture: This lecture examines the nature of logical definition. By working through the ideas of fundamentality, the genus and differentia, and definitional structure, Dr. Peikoff demonstrates the rules of logical definition.
Study Guide
This material is designed to help you digest the lecture content. You can also download below a PDF study guide for the entire course.
| What are the main differences between general philosophic questions about definitions and that of the specific field of logical definitions? |
| What is the basic cognitive need for definitions and what is their purpose? |
| What is a fundamental characteristic? |
| Why is the description of an entity not the same as a definition? |
| What is insufficient about using the etymology as a source of definitions? |
| How is a list of examples of a concept different from its definition? |
| What are the structural elements of a definition? |
| What is the vital role of the genus? |
| What rule of propositions allows you to check your definition? How? |
| Using an original bad and proper definition, explain the rule of fundamentality. |
| What is the rule of circularity? |
| What is the special rule for defining correlative concepts? |
| What is the purpose of the rule of negatives? |
| What is the application of the rule of obscurity in definitions? |
Q&A Guide
Below is a list of questions from the audience taken from this lecture, along with (approximate) time stamps.
| 2:27:31 | Are there definitional fallacies involved in the following form: “X consists of,” for instance “triangularity consists of a plane surrounded by three straight lines”? What about the expression “we call a Y an X”? |
| 2:29:13 | Please explain why the five rules for analyzing syllogisms are the only five rules needed to prove validity. |
| 2:30:59 | Why is the original syllogism valid if only one of its constituent syllogisms is valid when you translate it? |
| 2:33:40 | Please repeat the definitions of “essential characteristic” and of “definition.” |
| 2:34:56 | In “The Objectivist Ethics,” Miss Rand defines a value as “that which one acts to gain and/or keep.” Is she referring to all values rational or irrational in any morality? |
| 2:37:07 | Would you say that “color” is the genus of “red,” i.e., do ostensibly defined terms have genera? Is “existent” the genus of “entity”? |
| 2:40:17 | Would you discuss all irreducible primaries that can only be ostensibly defined? |
| 2:42:30 | Could you tell us where we can learn more about Elihu Palmer, John Dickinson, Samuel Adams, Thomas Paine, and Ethan Allen before your book comes out? Some of these men I’ve never heard of, and I wasn’t aware of any of them as men of ideas. |