[Lecture Five] Introduction to Logic
by Dr. Leonard Peikoff
Total Time: 2 hours, 49 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 begins the discussion and analysis of the syllogism as a logical form. Dr. Peikoff systematically works through the concepts of equivalence and mediation, and explains the parts of a categorical argument.
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 defining features of a syllogism? |
| What are the types of categorical statements about subjects in a syllogism? |
| What are the other parts of a categorical statement? |
| When the quantifier of a statement is “some” what does that mean? |
| What does the amphiboly fallacy imply for incorrect forms of categorical statements? |
| How does logic deal with singular subjects? |
| What is the goal of understanding whether a term in a proposition is distributed? |
| What is the difference between immediate and mediate inferences? |
| Indicate the three types of immediate inferences using examples. |
| What does it mean for two statements to have “logically equivalent” meaning? |
| What is the mediation that is achieved by “obversion”? |
| What is the mediation that is achieved by “conversion”? |
| What is the mediation that is achieved by “contraposition”? |
| In a categorical argument, what are the major, minor, and the middle terms? |
| What are the rules for the validity of a syllogism? |
Q&A Guide
Below is a list of questions from the audience taken from this lecture, along with (approximate) time stamps.
| 2:14:05 | Could you please again explain the risk for translating “unless”? For instance, “unless you study you will fail.” |
| 2:14:23 | Why is denying the consequent valid? |
| 2:16:56 | Is there a proof of the validity of affirming the antecedent? |
| 2:17:34 | Panic just set in. “P implies Q, P therefore Q.” That’s valid. “P implies Q, Q therefore P.” That’s affirming the consequent, invalid. Isn’t this the same as saying “A equals B, but B doesn’t doesn’t equal A.” |
| 2:19:11 | Is it permissible to ignore one of the premises in an argument if the argument can be proven valid without that premise? |