Glen, a student of mine, emailed me this question last week:

In class today you used the word “weak” in a way that seemed to contradict the logical usage of “weak”. You called a premise “weak”, but I thought “weak” and “strong” were only properties of whole arguments, not of premises.

It’s true, I did describe a premise as weak. I can understand how this might be confusing after hearing me harp about the correct logical usage of “weak”!

You’re right, the logical sense of the term “weak”, like the terms “strong”, “valid” and “invalid”, applies only to whole arguments, not to individual claims or statements. [See the tutorial course “Basic Concepts in Logic and Argumentation” for a complete discussion of these terms.]

But the terms “weak” and “strong” have another common meaning in argumentation that does apply to individual claims.
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There are plenty of books and courses that teach principles of logic and fallacies of reasoning.
Unfortunately, learning logic isn't enough to make you a good critical thinker.

There are two reasons for this:

1. Logic can tell you when a conclusion follows from a set of premises, but logic can't tell you whether or not you should believe the premises. Being a good judge of the truth or falsity of a premise is a function of your overall background knowledge, how much you know about the subject matter. Logic courses don't teach background knowledge. To be a good critical thinker about a given subject matter, you have to be knowledgeable about that subject matter.
2. Knowing principles of good logic doesn't guarantee that you'll follow them. People regularly, routinely, systematically deviate from idealized norms of rationality. There are branches of psychology devoted to studying and understanding this phenomenon.
   

Given this, what can you do to become a better critical thinker?

• Learn some logic. It's still very important to know what makes a good argument good and a bad argument bad.
• Learn about the psychology of human reasoning and decision-making. Under certain conditions we systematically make certain kinds of errors in judgment. Knowing these conditions can help you to avoid them (or at least mitigate their effects).
• Learn about the issues that matter to you. Improve your background knowledge. Read widely and critically from different points of view.
  

Future tutorial courses on this website will cover not only basic fallacies of reasoning but also the empirical literature on the psychology of reasoning and decision-making. But none of this can make up for ignorance.

A cartoon I did in Flash. It’s on the desktop background of my laptop right now...