Premises may begin with one of the following flags: because, due to, since, given that, owing to, as indicated by, in that, . . . Likewise, most conclusions have an identifying flag: therefore, consequently, thus, accordingly, hence, so, as a result, it follows that, . . . Usually the conclusion is the first or last statement in an argument. Sometimes, however, one has to search for the conclusion by asking oneself, ‘What is the author trying to convince me of?’ For example, examine the following argument and identify the premises, conclusion, and any extraneous statements.
Why should I have to study history? I am a scientist, I have more than enough to do already, I don’t like history, and history is irrelevant to science.
If one interprets the conclusion as ‘History is irrelevant to me,’ then the salient premises are ‘History is irrelevant to scientists’ and ‘I am a scientist.’ If one interprets the conclusion as ‘History is a waste of time for me,’ then the supporting premises are ‘History is irrelevant to scientists,’ ‘I am a scientist,’ and ‘Doing history would prevent me from doing something more worthwhile.’ The logic is valid, but some of the premises are dubious.
With deductive logic, each statement in the argument is either true or false. For the conclusion to be true, two critical preconditions must be met. First, the premises must be true. Second, the form of the argument must be valid. A valid deductive argument is one in which the conclusion is necessarily true if the premises are true. Validity or invalidity is totally independent of the correctness of the premises; it depends only on the form of the argument -- thus the term formal logic.
The following arguments demonstrate the distinction between the roles of premises and of logical form in determining the correctness of a conclusion:
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All dogs are cats. |
Valid form, but one false premise, so the argument is incorrect (although the conclusion happens to be true). | |
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All dogs are mammals. |
Valid form, true premises, so the argument is correct and the conclusion must be true. | |
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All dogs are mammals. |
True premises, but invalid form, so the argument is invalid and does not yield this conclusion. |
For these three examples, the reader already knows which conclusions are true and which are false without even evaluating the arguments. For scientific arguments, however, it is crucial that one considers separately the two elements -- premise correctness and argument form -- rather than accept or reject the argument based on whether or not the conclusion sounds right. Evaluation of premises requires subjective judgment based on local expertise. Evaluation of argument form, in contrast, is objective. With some practice and a few guidelines, the reader can avoid using invalid argument forms and recognize them in publications. Such is the main goal of this chapter.
Classification Statements
A building block of deductive logic is the classification statement; logicians use the term categorical proposition. The classification statement consists of a subject and predicate, and it states that
