# derivations

## Further help with Reductio

## Example of a difficult derivation

6/1/09

## Try your own derivations

## Roll your own derivations

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 ∼ & ∨ ⊃ ≡ ∀ ∃ ∴ (or use the palette to produce them)

## Help with Tutorial 25h

4/24/06

### Tutorial 25 Exercises h

## Help with Tutorial 25g

4/24/06

### Tutorial 25 Exercises g

## Help with Tutorial 25e

4/24/06

### Tutorial 25 Exercises e

## Help with Tutorial 25d

4/24/06

### Tutorial 25 Exercises d

## Tutorial 24: The restrictions on the quantificational rules

###### 12/25/13

### Skill to be acquired:

To understand how the various restrictions on the quantificational rules work to exclude certain kinds of invalid inferences

### Reading

Bergmann[2004] *The Logic Book* Section 10.1

### The Tutorial

#### The Rule of Universal Elimination UE

If a derivation contains a line of the form

n (∀<variable>)<scope> <any justification>

then a line of the form

## Tutorial 21: Existential Elimination

2013

### Reading

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.