PHIL 2030: Introduction to Ethics
Dr. Robert Lane
Lecture Notes: Friday January 16, 2015




[2.8.] The Provability Argument.


The next argument Rachels considers is an argument in support of full-blown moral skepticism. In other words, it is an argument in support of moral skepticism about everything, not just about a single issue (like infanticide, abortion, capital punishment, etc.). If full-blown moral skepticism is true, then no action whatsoever is objectively wrong ... not even rape, or killing innocent people for fun.


This argument is at RTD p.24:


1.      If there were any such thing as objective truth in ethics, we should be able to prove that some moral opinions are true and others false.

2.      But in fact we cannot prove which moral opinions are true and which are false.

3.      Therefore, there is no such thing as objective truth in ethics.


Remember the definition of “objective”: something is objective when it is the way it is whether or not anyone thinks, or feels, or believes that it is that way.


So this argument is trying to convince us that there is no fact of the matter – no “objective truth” – about whether any action is right or wrong. In other words, this is an argument for full-blown moral skepticism.



[2.8.1.] Valid or Invalid?


The Provability Argument is valid. It has the following argument form:


1.      If it were true that p, it would also be true that q.

2.      But it is not true that q. [This premise denies the q portion of the first premise.]

3.      Therefore, it is not true that p.


And any argument that has this form is valid. It does not matter what propositions you fill in for “p” and “q”; so long as both “p”s are replaced with the same proposition and both “q”s are replaced with the same proposition, the resulting argument will be valid. For example,


1.      If Atlanta is the capital of Georgia, then the moon is made of cheese.

2.      The moon is not made of cheese.

3.      Therefore, Atlanta is not the capital of Georgia.


This is a valid argument, even though one of its premises is false.



[2.8.2.] Rachels’ Criticism of the Provability Argument.


According to Rachels, the Provability Argument is unsound because the second premise is false.


He argues that we can prove moral claims to be true, and he gives a number of examples of moral opinions that he thinks we CAN prove to be true:

·         that a test was unfair

·         that “Jones is a bad man”

·         that “Dr. Smith is irresponsible”

·         that “A certain used-car salesman is unethical.”


We can prove that a test is unfair by showing that it has nothing to do with the class content, is too long for any student to finish, that it does not follow the study guide that the professor provided for the test, etc. And we can prove that Jones is a bad person by showing that he is a habitual liar, that he abuses his wife and children, that he tortures puppies for fun, and so on.


Discussion: is Rachels right to say that we can prove these claims to be true? Is there are difference between proving a moral claim and simply providing good reasons to believe it.


But even if Rachels’ criticism is right and the second premise of the Argument is false, this does not mean that the conclusion of the Provability Argument (full-blown moral skepticism) is false. As we saw previously, the fact that an argument is unsound does not imply that its conclusion is false. It might be an unsound argument for a true conclusion.


So the defender of full-blown moral skepticism needs to give a better argument if he or she is to convince us that that theory is true.



Stopping point for Friday January 16. For next time, study the lecture notes from today, and read and be prepared to discuss EMP sections 2.1 and 2.2.


Your first response paper, on ALL of EMP ch.2, is due at the beginning of class.



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