We began this course by defining an ARGUMENT as
a connected series of claims intended to ESTABLISH the TRUTH of one major claim,
the conclusion. We also acknowledged that SOME CLAIMS ARE TRUE (or false)
REGARDLESS of whether we (or anybody) believes them. Avoid confusing what is
believed with what is true: what is true might seem incredible ("Earth
rotates about its axis at 1000 mph.") and what is false may appear to be
obviously true ("The Sun revolves around Earth.")
1. In order to EVALUATE the credibility of claims we must develop two skills.
SKILL 1: DISTINGUISH a CONCLUSION FROM EVIDENCE offered in support of it.
SKILL 2: DISTINGUISH RELIABLE PATTERNS of reasoning from unreliable patterns of reasoning, so that we can separate good proofs from bad proofs.
2. GOOD arguments give us SUFFICIENT reasons to believe their conclusions.
The best ones actually provide COMPELLING evidence. When we evaluate arguments given what is claimed (and these claims are expressed in its assumptions or PREMISES) we have to ask two questions:
- DOES the conclusion FOLLOW FROM the premises?
- SHOULD the PREMISES be accepted as true?
Answering the FIRST question is not easy to do, but some FORMS of argument actually GUARANTEE that their conclusions are true. These FORMS of argument are so reliable that they PRESERVE TRUTH, whenever it is present. Such arguments are the most convincing, but what do they look like? We will learn how to recognize them by their PATTERNS OF REASONING and practice constructing some of our own.
RECOGNIZING TRUTH-PRESERVING FORMS of ARGUMENT and distinguishing them from non-truth-preserving forms helps us separate good from bad reasoning.
Sometimes we don't even have to ask the SECOND question: If we find that the conclusion of an argument does not follow from the assumptions of that argument EVEN IF THEY WERE TRUE, we don't have to inquire whether the assumptions are, IN FACT, true. We may simply REJECT the argument and ask for a better one.
ARGUMENTS that OFFER decisive, compelling PROOF for their conclusions are called DEDUCTIVE. Such arguments may or may not SUCCEED at this. Those that succeed in absolutely GUARANTEEING the truth of their conclusions are worthy of sustained consideration and acceptance. We call these DEDUCTIVELY VALID arguments. How VALID arguments accomplish this is a function of their form and not their content.
3. What is a VALID argument?
Answer: A VALID argument is one in which there is no possible way the premises could all be true and the conclusion false. To say that any argument could not possibly have true premises and a false conclusion is simply to say that it has the sort of FORM or STRUCTURE that does not permit us to derive or infer what is TRUE from what is FALSE.
Remember: Only arguments can be valid. Only claims are true (or false)
This definition of what constitutes a valid argument is complex, so let's look at a real argument. (What folllows is a deductive argument because it is supposed to provide convincing evidence for its conclusion, if its assumptions are true).
1. All liberals want to outlaw handguns.
2. Chuck is a liberal.
3. Therefore, Chuck wants to outlaw handguns.
IS THIS A GOOD ARGUMENT? Does it PERSUADE you to ACCEPT its conclusion? For a moment DISREGARD whether EACH claim is true. Would this PATTERN OF REASONING convince you to accept the conclusion, if each of its claims, in fact, WERE TRUE? It should. Here is why.
Notice that (3) cannot be true UNLESS (1) and (2) are TRUE, but if they were, then it (3) must be true.
EVEN IF both premises in this argument are false, the argument IS VALID because IF IT WERE TRUE that all liberals want to outlaw handguns and IF IT WERE TRUE that Chuck is a liberal, THEN IT WOULD HAVE TO BE TRUE that Chuck wants to outlaw handguns.
There are at least four basic standard argument forms I expect students to use when reconstructing their own or others' arguments:
- modus ponens,
- hypothetical syllogism,
- modus tollens, and
- disjunctive syllogism
Follow each link for more discussion and examples at Wikipedia. For my standard examples of deductively valid arguments see this HANDOUT.
For more examples, see this handout.