Unit 1: Introduction to Logic
The Science and Art of Correct Reasoning.
Table of Contents
1. Definition and Nature of Logic
Logic is derived from the Greek word 'Logos', which means both "thought" and "word" (the expression of thought in language).
It is often called the "Science of Sciences" because it provides the rules of reasoning that all other sciences must follow. It is also an Art, as it teaches us how to apply these rules in our daily discourse.
2. Scope: Formal vs. Material Truth
Logic is primarily concerned with Formal Truth. It doesn't care if the facts are true in the real world; it only cares if the conclusion follows logically from the premises.
- Formal Truth: Consistency. If the premises are accepted, the conclusion must follow.
- Material Truth: Correspondence. The statement matches the facts in the physical world.
3. The Structure of an Argument
An argument is not a "disagreement." In logic, an Argument is a group of propositions where one (the conclusion) is claimed to follow from the others (the premises).
- Premise: A proposition used as evidence or reasons.
- Conclusion: The proposition that is proven or supported by the premises.
4. Deduction and Induction
| Deductive Logic | Inductive Logic |
|---|---|
| Moves from General to Particular. | Moves from Particular to General. |
| Claims 100% certainty (Validity). | Claims Probability. |
| If premises are true, conclusion must be true. | Conclusion goes beyond what is in the premises. |
5. The Fundamental Laws of Thought
Logic rests on three bedrock principles established by Aristotle:
- The Law of Identity: A thing is what it is (A = A).
- The Law of Non-Contradiction: A thing cannot be both A and not-A at the same time and in the same sense.
- The Law of Excluded Middle: A thing is either A or not-A; there is no third middle ground.
Exam Focus
- Define Logic: Be ready to explain why logic is both a science and an art.
- Truth vs. Validity: Validity is a property of arguments; Truth is a property of propositions. An argument can be valid even if its premises are false.
- Laws of Thought: You will often be asked to name and explain the three laws of thought.