Chapter 6  

Evaluating Argument I:  Deduction

The intuitions which guide us while recognizing the superiority of a particular argument as compared to one that is obviously relatively weak often fail us when we attempt to analyze arguments that are more closely matched.  There exists a need for a theory that will guide us in this undertaking.  There is a basic distinction between the structure of an argument and the components that constitute the argument.


Analogy of the house:  There are two very distinct elements that are involved in building a house.  The house must be constructed by an individual who can put all the pieces of the house together properly.  This contractor must understand the various building codes the limitations of the material with which (s)he is working, the proper nailing techniques, roofing and foundation construction, etc.  Although this contractor is the most talented of contractors and understands and is able to execute the to the finest detail all that is required of the project the subsequent project will be a dismal failure if the contractor uses substandard material or material that is faulty.  In the same light a contractor can purchase and install nothing but material of the highest quality.  The finest roofing, siding, appliances and the like are useless if the contractor doesn’t know how they are to be applied.  If the formal training of the contractor is lacking the house will be substandard.  This analogy attempts to illustrate a basic distinction between the structural feature (formal) and the material feature (informal) used in the construction of an argument.  Design factors/Material factors.


At this point the text gets a little ahead of itself and introduces some concepts that require the introduction of some basic information.


Deductive and Inductive reasoning:  2 main argument designs – Whether an argument is inductive or deductive determines the manner in which we approach the evaluation of the argument.


Deduction – As stated earlier an argument is a set of propositions so arranged that one of the propositions (conclusion) is said to depend or follow from the other propositions (premises).  In a deductive argument the premises are intended to make the conclusion

Inevitable.  That is, the premises of a deductive argument are intended to make the conclusion inevitable.  There is also another important point about the conclusion of a deductive argument.  The conclusion of a deductive argument never goes beyond the information provided in the premises.  The conclusion of a deductive argument never claims more than that which was offered as evidence.  A formal deduction system deals with only the form or pattern of the argument and never considers whether the propositions offered in support of the conclusion are “true” or “false”.  A formal deduction system deals only with the form of the argument not the content.  This brings  us to a discussion of the terms valid and invalid.  A valid argument is one in which the truth of the premises guarantees the truth of the conclusion.  It is impossible for a valid argument to have true premises and a false conclusion.  A valid argument is an argument so constructed that if the premises are true then the conclusion must be true.  If the argument is so constructed that even possibly true premise could produce a false conclusion we claim that the argument in invalid.  However, validity has nothing to do with the truth of falsity of the evidence.  All that is being considered at this point is how the propositions are being used in support of the conclusion.  It is possible for a valid argument to have a false conclusion if one of the premises is false.  There are many different considerations.  It is also possible to have an invalid argument with false premises and a true conclusion (This usually drives people crazy) But for our purpose all observations about whether or not an argument is valid or invalid.  IMPORTANT POINT – Arguments are either VALID or INVALID never, for the purpose of this class, say “that’s a true argument”.  Propositions are statements that either true or false.  A conclusion is a proposition. Therefore conclusions and premises are either TRUE or FALSE.  For the purpose of this class never say, “That’s a valid claim or an invalid conclusion”.


Induction -