# derivations

## Tutorial 21: Existential Elimination

Topic
Logical System

2013

Bergmann[2004] The Logic Book Section 10.1.

### The Tutorial

Existential Elimination (often called 'Existential Instantiation') permits you to remove an existential quantifier from a formula which has an existential quantifier as its main connective. It is one of those rules which involves the adoption and dropping of an extra assumption (like ∼I,⊃I,∨E, and ≡I).

The circumstance that Existential Instantiation gets invoked looks like this.

## Tutorial 20: Existential Introduction

Topic
Logical System

2013

Bergmann[2004] The Logic Book Section 10.1.

### The Tutorial

There is a rule for adding a Existential Quantifier, Existential Introduction (also commonly known as 'Existential Generalization'). This permits the step illustrated by the following proof fragments.

## Tutorial 19: Universal Introduction

Topic
Logical System

2013

### Skill to be acquired:

To learn the rule for adding a Universal Quantifier.

Bergmann[2004] The Logic Book Section 10.1.

### The Tutorial

There also is a rule for adding a Universal Quantifier (often called 'Universal Generalization'). This permits the step illustrated by the following proof fragments.

The rule is

## Tutorial 18: Universal Elimination

Topic
Logical System

2013

### Skill to be acquired:

To understand the concepts of scope, free, bound. To meet substitution and the rule for removing a Universal Quantifier.

Bergmann[2004] The Logic Book Section 10.1.

## Tutorial 15: Elementary derivations in predicate logic

Topic
Logical System

2013

### The Tutorial

The sentential rules of derivation carry over unchanged into Predicate Logic

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## Predicate Logic: 15 Tutorials

Topic
Logical System

There are many valid arguments which cannot be shown to be valid using sentential logic alone. For example,

Beryl is a philosopher.
All philosophers are wise.
Therefore
Beryl is wise.

Topic
Logical System

2013

You may have derivations of your own that you wish to try. Just type, paste, or drag and drop, them into the panel, select your derivation, and click 'Start from selection'.

[Often copy-and-paste won't work directly from a Web Page; however, usually drag-and-drop will work!]

You will need to use the correct logical symbols. Here they are

F ∴ F & G ∼ & ∨ ⊃ ≡ ∀ ∃ ∴

Supplementary: Why are we doing all these derivations anyway? admin Sat, 01/11/2014 - 01:50
Topic
Logical System

3/16/06

So that we can show certain arguments to be valid.

The focus of the course lies with the validity and invalidity of arguments. Now, invalidity can be established by counter-example (by producing an interpretation under which all the premises are true and the conclusion false, at the same time). But validity is a different matter. And the usual approach is to have rules of inference and to do derivations.

Topic
Logical System

## Replacement or Rewrite Rules

6/21/07

Rewrite Rules

Tutorial 10: Common Inference Patterns and Rewrite Rules admin Sat, 01/11/2014 - 01:50
Topic
Logical System

2013

### Skills to be acquired

Becoming familiar with common inference patterns and being able to use them via three new rules of inference and via rewrite rules. This helps with assessing ordinary everyday reasoning such as that found in the law, in newspapers, in advertisements, etc.