Deductive Versus Inductive Strength Of Arguments
 
    A correctly formed deductive argument is one whose form is such that the conclusion follows with logical necessity or certainty from the premises. In other words, if the premises are true, the conclusion must also be true. Another way of describing the relationship between the premises and the conclusion of a valid (or correctly formed) deductive argument would be to say that it is impossible for such an argument to have true premises and a false conclusion. One could not accept the premises and deny the conclusion without contradicting oneself. For example:

Since all senators in the US Senate are at least 35 years old.

and John Morgan is a US senator,
Therefore, John Morgan is 35 years or older.

The conclusion of this or any deductive argument simply spells out what is already implicit in the premises. If one can get others to accept the crucial premises, which already include the conclusion, then the arguer's work is done. The argument is indeed so strong that the conclusion cannot be denied.

    A very effective strategy that is sometimes employed in argumentation is that of constructing an argument in this deductive way, so that the conclusion is, in effect, accepted when the crucial premises are accepted. One would then have a foolproof argument for one's claim. Moral arguments are often presented in this deductive form. Consider the following example:

Since sexist practices are wrong,

and the use of male-dominated language is a sexist practice or tradition,

Therefore, the use of male-dominated language is wrong.

If the arguer can get his or her opponent to accept the first premise, there is little likelihood that the conclusion can be denied. This is not to say that there cannot be any dispute about the meaning of sexism. The point is that the crucial and most controversial premise here is most likely to be the first one, and if it is accepted, the deal, in effect, is closed.

    An inductive argument is on in which the premises are supposed to provide some evidence for the truth of the conclusion. However, the conclusion of an inductive argument does not follow with logical necessity or certainty from the premises, even if all the premises are true, because the conclusion is not already contained in any of the premises. Therefore, in contrast to the deductive argument, the truth or acceptance of relevant premises in an inductive argument does not force or guarantee the truth of its conclusion. For example:

Since John Morgan is the most popular Democrat in the Senate,

and he is personally very charming and articulate,

and he has moved to a politically moderate position on most issues,

and he always easily wins reelection to his Senate, seat,

and he is in great demand on the speaking circuit,

and he is often mentioned by prominent journalists and other Democrats as a possible presidential candidate,

Therefore, Senator John Morgan will be chosen by the Democrats as their next presidential candidate.

The conclusion of this or any inductive argument is at best only probable, because the conclusion makes a claim that goes beyond the evidence provided in the premises. It is quite possible that an inductive argument might fail to take into account crucial information that would be relevant to the truth of the conclusion. For example, if Senator Morgan is uncertain or even negative about running for the presidency, that fact could obviously affect the truth of the argument's conclusion.

    Most of the arguments that we encounter in our everyday world will be inductive arguments. For that reason, most of them will not exhibit the kind of force that a deductive argument would have. Nevertheless, it is sometimes possible to reformulate an inductive argument in such a way that it takes on a form and the power of a deductive one. Consider the following inductive argument:

Since you underreported your income on your income tax form,

and many of us would say that such underreporting is cheating the government,

and persons who underreport their income cause taxes to be raised for all of us,
Therefore, it was wrong for you to have underreported your income on your tax form.

All of these premises would probably be accepted as true, but the conclusion does not necessarily follow from the premises as stated. It is possible to accept the premises but reject the conclusion. However, if we change this argument into a deductive one and use a premise that is likely to be accepted that implicitly contains the conclusion, acceptance of the conclusion would be guaranteed. For example:

Since all stealing is morally wrong,

and underreporting your income on the tax form is a case of stealing from the government,

and you underreported your income on your tax form,
Therefore, it was morally wrong for you to have underreported your income on your tax form.

If one were to reformulate the original inductive argument in this deductive form, it would be a much more powerful argument, because if the premises of this deductive formulation of the argument are accepted as true, the conclusion must be accepted also.

 

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["Attacking Faulty Reasoning", By T. Edward Damer]