In my last Friday Fallacy post, I looked at the fallacy of denying the antecedent. There I discussed conditional statements, statements of the form “if P then Q”. Examples would be statements such as “if it is raining then the grass will be wet” or “if the US had not shot Bin Laden then he would still be alive”. I noted that conditionals have an “antecedent” and a “consequent”. In the above examples the antecedent is the claim “it is raining” or “the US had not shot Bin Laden”. In a conditional statement one talks about what occurs if the antecedent is true. The consequent is what is said to be true if the antecedent is correct; in the examples above the consequent is the proposition “the grass will be wet” or “he would still be alive” respectively. [Read more…]
By Matthew Flannagan. Here in New Zealand, I am often told by evangelical leaders that we now live in a post-modern society, which has moved beyond “arguments” and that Apologetics is an outdated “modernist concept.” They say we need instead to “tell the story” so that people will see the “meta-narrative of scripture”—whatever exactly that means.
Last night, Madeleine and I were invited to a Christmas function for new lawyers, organised by the Law Society, the professional association for lawyers in New Zealand. The function was in a major law firm in central Auckland’s business district. So I was right in the thick of the up-and-coming legal professionals in New Zealand. [Read more…]
This week I will look at the fallacy of denying the antecedent. Before I can elaborate exactly what is involved in this fallacy, it is important to introduce and analyse some valid arguments that are superficially similar.
One of the very first valid inferences one learns in logic is modus ponens. To use the well worn example that was repeated ad nauseam when I was learning logic (and one I probably bored my students with too) a paradigmatic example of modus ponens is,
1. If it is raining then the grass will be wet.
2. It is raining;
3. The grass will be wet.
Put more abstractly, a modus ponens has the form:
1’ If P then Q.
Modus ponens proceeds with the first premise contending that a conditional statement is true. A conditional statement is a statement about a hypothetical situation; in this case the claim is “if it is raining then the grass will be wet”. Notice that for this conditional to be true, it does not have to actually be raining. On a sunny day it is still true that if it starts raining the grass will be wet. A conditional statement tells us what will be the case if some other thing or event is the case – not what actually is the case. [Read more…]
In my post on Assessing Arguments I noted that a valid argument is one where it is impossible for the premises to be true and the conclusion false. I gave the following example:
Premise: All men are under 10 feet tall;
Premise: John is a man;
Conclusion: John is under 10 feet tall.
This argument is valid because it is impossible for both premises to be true and the conclusion false. If it is true that all men are under 10 feet tall and if it is true that John is a man then John cannot be over 10 feet tall. If it were possible for John to be over 10 feet tall then the first premise would mean he could not be a man, which, of course, would contradict the second premise. [Read more…]
In the discussion following last week’s Fallacy Friday topic, Ad Populum, LJ asked about the ad ignorantiam fallacy. In particular she wanted to know about its relationship to creationism. I suspect LJ was being sarcastic but despite this it is worth exploring this issue a bit.
First, we should recall that a fallacy is not simply a false position on some subject. The fact that someone expounds a mistaken view in theology, philosophy, science, law, ethics or any other subject does not necessarily mean that that person has committed a fallacy. A fallacy is a common mistake in reasoning. A fallacy occurs when one attempts to infer a conclusion from a premise in an invalid manner. I noted in Assessing Arguments that an argument can be valid even if it has false premises and false conclusions. So the fact that someone believes in or propounds contemporary evolutionary theory or creationism or something else does not, in and of itself, mean that that person has committed the ad ignorantiam fallacy. [Read more…]