What is critical thinking?
Essentially, critical thinking is making sure you have good reasons for your beliefs.
What does that mean?
When we distinguish between good and bad reasons for believing something, we are exercising our critical thinking. What does it mean to distinguish between "good" reasons from "bad" reasons to believe something? Good in this usage has nothing to do with the question of morality or ethics. It isn't morally "right" or morally "good" to believe something on the basis of believing a claim for "good reasons" or bad. Rather, a good reason for believing a claim is one that is probable, or the reason that gives a belief likely to be true. The best reasons for belief make that belief almost or definitely certain.
Why does this matter?
Since we are nominally rational creatures, we want our beliefs to be true. Rational people want to have true beliefs, not false beliefs. The best way to be rational in this way is to only form beliefs when we have good reasons for them.
What is an argument?
An argument is a set of statements that taken together comprise a reason to believe a further statement.
The statements that are the reason for the argument are known as the premises. The statements that those premises give you a reason to believe we would call the conclusion. A good argument is one in which the premises give you a good reason to believe the conclusion. Good arguments support their conclusion. Bad arguments don't support their conclusion.
A key part of critical thinking is learning to evaluate arguments to determine whether they are good or bad, or rather, whether their premises support their conclusions.
What are Deductive and Ampliative Arguments?
Suppose that you and your friend are talking about who is going to be at tonight's party. She says to you quite confidently "Monty won't be at the costume party tonight." You're not sure whether to believe her, so naturally you follow up by asking, "Why do you think so?" In response, let's say she makes two different arguments for why Monty won't be at the part.
Both arguments she makes constitute "good" arguments, in that they both give valid reasons for believing the conclusion for why Monty won't be at the party tonight
First Argument
Premise 1: Monty is really shy.
Premise 2: Monty rarely goes to parties.
Conclusion: Monty won't be at the party.
Second Argument
Premise 1: Monty is in Beijing.
Premise 2: It is impossible to get here from Beijing in an afternoon.
Conclusion: Monty won't be at the party.
Evaluating the Arguments
First, let's consider the Beijing argument. If both of those premises are true, then the conclusion must be true because the premises guarantee the conclusion. When the truth of the premises guarantees the truth of the conclusion, we call this a deductive argument. One need only consider the given premises to reach a conclusion that an argument is true or false.
Now let's consider the argument about Monty's shyness. Even if both premises are true, the conclusion might possibly still be untrue. Monty could decide to suspend his policy of not going to parties and being shy and decide "Today's the day!" and make an appearance at the party. Unlikely? Sure. But it is possible. The truth of the premises do not guarantee the truth of the conclusion. Arguments like this are called ampliative argument. The argument is probable, but not conclusive. Ampliative arguments can be very good arguments...they're just not deductive.
When you're evaluating an argument, it can be important to know whether the argument is supposed to be deductive or supposed to be merely ampliative.
If the argument is supposed to be deductive, but careful analysis of the argument reveals that the premises don't guarantee the conclusion, that's often a good reason to reject the argument as a bad argument.
In an ampliative argument, to notice that the truth of the premises don't guarantee the conclusion's truthfulness is simply to notice that it's an ampliative argument. If you were to point out that the conclusion to the premises of ampliative argument could be false, you'd be missing the point. It's taken for granted in an ampliative argument that the conclusion isn't guaranteed. The point of an ampliative argument is to point out that the conclusion is probable.
Knowing what kind of argument is being made is essential to deciding which tools to use to evaluate that argument.
To Sum This All Up
Critical thinking is making sure we have good reasons for our beliefs. We understand that a good reason is one that makes the argument probable. An argument is a set of statements we call premises that together comprise a reason for another statement, called a conclusion. In a good argument, the premises support their conclusions, which is to say they give you a good reason for believing the conclusion because they make it probable. A deductive argument is one where the conclusion is guaranteed by the premises if they are true. An ampliative argument is one where the premises don't guarantee the conclusion, but they do make it probable, so they can provide you with a good reason for believing the conclusion.