Critical Thinking with Argument Maps

By Dave Kinkead, University of Queensland Critical Thinking Project


Evidence can help know the truth of the premises in an argument, but the value of argumentation lies in the way we can infer the truth of the conclusion. So it’s worth delving a little deeper into this.

An argument is simply a connected series of statements (called premises) used to establish the truth of some other statement (called the conclusion). Depending on the logical structure of an argument – of how the different propositions connect to each other – we can determine the truth (or likelihood of the truth) of the conclusion based on the truth of the premises.

If the premises being true makes the conclusion more likely to be true, then we can say that the premises support the conclusion. The degree of this support provides us with an important criterion for judging the quality of an argument.

The key insight of reasoning then, is that we can know the truth of the conclusion without ever having experienced or witnessing it before. In short, arguments give us reasons to believe a claim is true, in a way not linked to the identity, social position, tenor, or rhetorical ability of the person making the claim.

That’s all a bit abstract so let’s make it more concrete – consider these three claims:

  1. Either Fred ate my chocolate or Bob did.
  2. Bob is lactose intolerant and hates all dairy foods.
  3. Therefore Fred ate my chocolate.

I may not have any access to direct evidence (via empiric observation or authoritative source) about the truth of #3 but if I do know that #1 and #2 are true, then I am justified in believing #3 is true because #1 and #2 being true makes #3 very likely to be true.

This process of moving from premises to conclusion is called inferring and the quality of the inference determines how justified our belief in the conclusion is. If all the premises being true makes the conclusion more likely to be true, we can say that the inference is a “good” one.

So let’s look at our previous argument map …

As it stands, it is a weak argument because the link between abortion and murder hasn’t been made. Making this connection more explicit makes for a stronger argument.

Note here that if the premises are true, then the conclusion must be true. In fact, given the meaning of the words, it would be impossible for the conclusion to be false while all the premises are true.

Note also that just because the inference is very strong, this tells us nothing about whether or not the premises are true. For that we need evidence.

Next lesson …

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