Inductive and Deductive Logic
Well, normally life isn't like that, and you have to be on your guard when you have any sort of debate or discussion to stop yourself being fooled to a particular point of view. Particularly, of course, when listening to political debate!
However, there is one sort of argument where the conclusion always follows on from the premises, and this is called a deductive argument. In other words, if the premises are true, then the conclusion must also be true. Here is an example of an argument of this form:
Deductive Reasoning
1 - All men are mortal
2 - Aristotle is a man
3 - (Therefore), Aristotle is mortal
In the argument above, if the premises are true (lines 1 and 2) then the conclusion necessarily is true also. Using formal logic, this can be 'translated' into sentences that reveal the argument form behind the argument, and create an example of a form of argument that must be true if the premises are. Read the articles on sentential and predicate logic for more information on these which will also look at the concepts of an argument being valid and sound.
Inductive Reasoning
By contrast, most arguments we make are inductive, and here the conclusion is not necessarily entailed by the premises, though it can be exceedingly likely.
Inductive reasoning will tend to move from specific examples where we think we see patterns/regularities, to the generic case or to predict future events.
A classic example is 'the sun has always risen every morning. So the sun will rise tomorrow'.
OK, so it is extremely likely the sun will rise tomorrow, however it is not necessarily entailed that this happens from the premise, because it could be that the sun won't rise tomorrow, however improbable.
A classic case of when inductive reasoning goes wrong is this:
I have only ever seen white swans, therefore all swans are white.
However, those of us who live in areas where white swans are the only swans in town now know that in other parts of the world there are actually black swans too, so this is false, though before international travel many people really would have thought all swans were white. The absense of a counterexample tends to make us think we are right, but this is not always the case.
In other words, reasoning from all cases we have seen, or particular instances, to the absolute (universal) is not a logically (that is to say here, deductively) valid form of argument.
We all reason inductively yet the fact that it is not strictly valid in the above sense creates the problem of induction as discussed by Hume, leading to the formulation of a so-called practical scepticism.
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