Contemporary Western philosophy treats arguments as coming in two main types, deductive and inductive. The basic distinction and difference will be mentioned here.
Deductive arguments are arguments in which the premises (if true) guarantee the truth of the conclusion. The conclusion of a successful deductive argument cannot possibly be false, assuming its premises are true. This is what it means to label an argument as “valid” in logic. The form or structure of a deductive argument is the essential aspect to consider. Somewhat counter-intuitively, the premises do not need to be true for the conclusion to be true.
Arguments are a linguistic representation of an inference. So, using slightly different terminology, we can define deductive inferences. In a successful deductive inference, the premises and the denial of the conclusion constitute an inconsistent set of statements. An alternative way to describe the same relation: in a successful deductive inference, the truth of the premises makes the falsity of the conclusion logically impossible. A successful deductive inference is valid.
Deductive Example
1) All dogs are mammals.
2) All mammals breathe air.
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SO: All dogs breathe air.
Inductive arguments are arguments with premises which make it likely that the conclusion is true but don’t absolutely guarantee its truth. Inductive arguments are by far the most common type of argument we see in our daily lives. We can assess inductive arguments along a spectrum of successful (stronger) to unsuccessful (weaker). The more successful (stronger) argument suggests that the premises mean the conclusion is probably true, with a high degree of likelihood. It is important to remember that inductive arguments can never fully guarantee the truth of the conclusion.
Using slightly different terminology, we can consider inductive inferences, referring to the actual thinking process in someone’s mind. In a successful inductive inference, the truth of the premises makes the falsity of the conclusion possible, but unlikely. Inductive inferences can be evaluated as “stronger” or “weaker” depending on the probability.
Inductive Example
1) The Interstate Bridge is regularly inspected by qualified engineers.
2) Vehicles have been driving over it for years.
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SO: It will be safe to drive over it tomorrow.
One thing that makes applying the distinction between deductive and inductive arguments a bit tricky is this: we can’t look only at the premises OR only at the conclusion. Instead, we need to focus on the relationship between the premise(s) and the conclusion to tell what kind of argument we have.
A further contributor to trickiness: we can’t be distracted by the question of whether the statements are true or false. To classify an argument as deductive or inductive, we need to grant that the premises are true in a hypothetical way. We have to ask the question, “If those premises were true, would it be IMPOSSIBLE for the conclusion to be false?” If so, it is a deductive argument. Or “If those premises were true, would it be UNLIKELY, but still possible, that the conclusion is false? If so, it is an inductive argument.
As an example, consider this valid deductive argument:
1) All clouds are made out of spun sugar.
2) Anything made out of spun sugar is high in calories.
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SO: All clouds are high in calories.
This argument is deductively successful because the truth of the premises would make the falsity of the conclusion impossible. Odd, isn’t it?
Some arguments are presented with premises missing. In those cases, the determination of deductive or inductive will depend on how that premise is filled in.
For example: I had an apple for lunch, so I had something healthy!
| Deductive | Inductive |
| P1) I had an apple for lunch.
P2) All apples are healthy. (implied) SO I had something healthy |
P1) I had an apple for lunch.
P2) Most apples are healthy. (implied) SO I had something healthy |
Exercise: Deductive or Inductive?
Determine if the following arguments are deductive or inductive. It is a good idea to put the arguments in standard form first, so you are clear about the relation between premises and conclusion.