# Straw Man

### A Lesson in Logic

This is a common fallacy, and I’ve been waiting for a chance to talk about it. The basic idea of the Straw Man, (Called the Aunt Sally in the UK), is reconstructing another person’s argument in a weaker form than they made it, making it easier to dismantle. Creating a straw man to jab your logic at, instead of a real person. Today’s guest straw man fallacy is provided by Clint Eastwood. I’m a big fan of his, and I’m excited to get to include him in a post, but less excited about the context.

# False Dichotomy

A Lesson in Logic

I don’t have a specific order I want to look at the fallacies in, at least not yet. I just finished my Master’s degree, so it’ll be a week before my life takes on any semblance of organization again. All that aside, I decided to let it be dictated by the news. Logical fallacies happen everywhere that people make arguments, and learning them will help you sort out good arguments from bad, and help you understand why an argument that feels wrong is (provided it is). Get ready to learn the first logical fallacy, the false dichotomy

# Being Charitable

### A Lesson in Logic

Before getting into fallacies, we need to have a talk about the principle of charity. This has nothing to do with giving other people money for good reasons, though there’s a principle of charity for that too. No, when we talk about being charitable in reference to an argument, it’s about cutting other other person some slack, and filling in pieces of their argument for them. Interpreting someone’s argument charitably means representing their argument as strongly as possible, even if they haven’t been clear about all of those elements. Scratch that, especially then.

# Making It Work

## A Lesson in Logic

So now you’ve learned all of the rules of inference, all of the substitution rules, and the rules concerning assumptions. You know what an argument is, what a proposition is, and how to do a proof. When it comes to deductive logic, you’re sitting pretty. But what can you really do with this? How does it work in the real world? I’ve been talking about how each piece works, but it’s time to put it all together. If you haven’t learned all of these things from the logic posts over the past few weeks, don’t sweat it, but come in and learn how you can use logic in your everyday life.

# Working Indirectly

### A Lesson in Logic

Last week I covered using assumptions to discover conditionals in what’s called a Conditional Proof. This week we’ll be using assumptions in a different way, to generate contradictions. This is also the very last rule. In two-value propositional logic, if you’ve been following along and have mastered all of the rules, this is the last one You will have completed the set and be the master of logic! Well, this particular kind, anyway. So, let’s make some contradictions!

# Assumptions in Proofs

### A Lesson in Logic

This week I’m going to look at how to make assumptions in proofs. There are two ways to use assumptions, called a conditional proof and an indirect proof. Today I’ll look at the first, and show you how it works.

# Rule Index

We’ve finished all of the rules but the two proof applications, so I thought I would do a quick rundown of how each rule works. No guest arguments today, this is going to be a lightning round of symbols and links to help you mind your p’s and q’s.

# DeMorgan’s

### A Lesson in Logic

Here we are, our final substitution rule, and the only one named after a person, rather than a function. Named after Augustus DeMorgan, this rule, which is actually two rolled into one, and can look a little tricky at first, but is really useful.

# Exportation

Not just what your country’s economy is based on, nor a criticism of free trade, but a new substitution rule. Our second last, in fact. If you’ve been following along, you’ve learned almost all of the essential rules of two-value first order logic. If you haven’t been, then welcome to Concept Crucible, and you can click here to get caught up. Now, on to exportation, starring my special guest, the friendly neighborhood Spiderman!

# Biconditional Exchange

### A Lesson in Logic

It’s a long weekend here in Canada, and I’m close to finishing a draft, so here’s a quick and straightforward substitution rule for you, so you can get on with your weekend. Remember the biconditional? The one that looks like ≡? No? I’m not surprised. Today’s the day it gets to shine.