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.
In this post I want to note an important feature of this type of argument. There is an obvious link between the first and the second premises. The first premise makes a claim about all men; it identifies a class of beings, those we call men, and it attributes a property to members of this class. The second premise identified John as a man – a member of the class mentioned in premise one. The fact that both premises refer to the same class of beings is important to the validity of the argument because John is a member of the same class that is stated to be under 10 feet tall; the conclusion, that John is under 10 feet tall, cannot be logically avoided. This argument is valid because the word ‘man’ refers to the same thing in both premises. If the word ‘men’ in the first premise did not refer to the same class John is said to be a member of in the second premise then the argument would not follow.
This brings me to this weeks fallacy: the fallacy of equivocation. Sometimes the same word has more than one meaning. The fallacy of equivocation occurs when a person uses the same word in different premises but not the same meaning. Because the same word is used the argument appears valid but when one looks at what the word actually refers to in each premise, the argument is not valid. A obvious example is as follows:
1. Money put in a bank will earn interest;
2. A bank is the body of land at the side of a river;
3. If you want to earn interest on your money you should put it in the body of land at the side of a river.
This silly example shows quite nicely how the fallacy works. Both premise 1. and premise 2. make reference to a bank; at a superficial glance the argument might appear valid if what the first premise says of a bank is true and if what the second premise says of a bank is true. The problem is (and in this example it is fairly obvious) the word ‘bank’ in each premise has a different meaning. In the first premise it refers to a financial institution, in the second it refers to a body of land at the side of a river. When one looks at what the terms actually refer to, as opposed to their verbal form, the argument is invalid.
In this example the fallacy is obvious; the conclusion is so outrageously silly and the premises so uncontroversial that the mistake is obvious. However, in other cases the fallacy can be more subtle.
Here is an example I recently spotted on Common Sense Atheism’s “Sam Harris vs. William Lane Craig debate review (part 1)“
“Either God has good reasons for his commands or he does not. If he does, then those reasons (and not God’s commands) are the ultimate ground of moral obligation. If he does not have good reasons, then his commands are completely arbitrary and may be disregarded. Either way, the divine command theory is false.”
It is worth noting this argument is not unique to Luke Muehlhauser. The argument appears over and over in Ethics text books discussing the issue of God and Morality. It is used both by prominent philosophers Wes Morriston and Walter Sinnott-Armstrong. This argument is typically believed to be decisive against divine command theory; however, I think it commits the fallacy of equivocation. Consider the argument:
 Either God has good reasons for his commands or he does not;
 If he does have good reasons then those reasons (and not God’s commands) are the ultimate ground of moral obligation;
 If he does not have good reasons then his commands are completely arbitrary and may be disregarded;
 Either God’s commands are not the ultimate grounds of moral obligation or they may be disregarded.
The problem here is that the word ‘reasons’ is ambiguous. William Wainwrightnotes that the word ‘reason’ can be used in two different senses. The first is aconstitutive sense. This sense occurs when one explains one thing by identifying it with another. Consider the case where a person affirms that thereason water has certain properties is because it is H2O. When the word ‘reason’ is used in this sense, the use of the word ‘reason’ denotes a relationship of identity; one is saying that water is identical with H2O.
The second sense is a motivational reason. This sense occurs when one identifies factors that motivate a particular action. For example, the reason I feed my daughter is because I love her and feeding her in a important part of her flourishing. This sense is more psychological and does not refer to a relationship of identity.
It is important to note that these two senses are not the same as the following illustration demonstrates. My son Noah fills a glass with water. If we ask what the constitutive reason was for his action, the answer would be that he filled the glass with water because he filled the glass with H2O. If we ask what the motivational reason was for his action, the answer would be that he wanted a drink. Yet his wanting a drink does not constitute water; likewise, water being H2O is not the motivational reason why he wanted a drink.
Now turning back to the premise , “Either God has good reasons for his commands or he does not”, when the objector makes this claim he could be talking about whether God has motivating reasons for his commands or he could be asking if there are constitutive reasons for his commands.
If the objector is referring to a motivating reason then the third premise of the argument is correct; if God has no motivating reasons for commanding as he does then his commands are arbitrary. To avoid the conclusion that God’s commands are arbitrary one would have to concede that God has motivating reasons for issuing them.
The problem is that on this sense of ‘reason’, the second premise of the argument is false. If God does have motivating reasons then it does not follow that those reasons (and not God’s commands) are the ultimate ground of moral obligation. This is because when people claim that God is the ultimate ground of moral obligation, they typically mean that moral obligations are identified with God’s commands. In other words, they claim that God’s commands are theconstitutive reason for our moral obligations. So the fact that one has amotivating reason for an action does not mean that these reasons constitutethe action. I noted this in the example I gave above; the fact that Noah has amotivating reason to pour water into a glass does not mean that these motivations constitute him pouring water into the glass. What constitutes water are H2O molecules, not motivations.
This brings us to the second option. Perhaps in premise  the objector is not referring to a motivating reason, rather he is referring to some kind of constitutive reason. This might enable the objector to sensibly claim that the second premise is true. If something other than and prior to God’s commands is identical with moral obligations then God’s commands will not be the ultimate ground of moral obligation. The problem is that if this is what is meant by the word ‘reason’ then the third premise is false.
Even if God does not have constitutive reasons for his commands, he could still have motivating reasons for issuing them. If God has motivational reasons for issuing the commands he does, such as concern for the welfare of others, then God’s commands are not arbitrary.
So this argument commits the fallacy of equivocation. Professional philosophers, even very brilliant professional philosophers, mistakenly consider the argument decisive because the word ‘reason’ is used in both premises.
This example is admittedly a bit technical but it demonstrates how the fallacy of equivocation can be beguiling. The key point is that when one is offering an argument for a position or discussing an issue, it is important to be clear as to what the terms being used mean and to ensure that this use remains consistent throughout the argument.
 William Wainwright Religion and Morality (Aldershot: Ashgate Publishing, Ltd, 2005) 91.
Every Friday I publish another post in my Fallacy Friday series. To navigate the whole series, use the Fallacy Friday tag.