Arguments in Action

Which is more reliable deductive or inductive reasoning?

Deductive reasoning aims to construct an argument such that if the premises are true then the conclusion must be true. With inductive reasoning, even if the premises are true the conclusion can be no more than highly probable. The instinctive response, then, is to say that deductive reasoning must be more reliable. However, things are not so straightforward. Consider the following two arguments:

Argument one (deductive)
All swans are white. Therefore, if there is a swan on the river it will be white.

Argument two (inductive)
Every swan I’ve ever heard about has been white. Therefore, if there is a swan on the river it will be white.

The first thing to notice is that it is possible to be much more confident about the truth of the premise in the inductive argument. Secondly, if we ask how we know that the premise in the deductive argument is true then we will presumably have to rely on:

Argument three (inductive)
Every swan I’ve ever heard about has been white. Therefore, all swans are white.

When evaluating arguments one and two any doubt about the sufficiency of the premise in the inductive argument is mirrored by doubt about the acceptability of the premise in the deductive argument.

Indeed, it is worse than that. Argument three is an inductive generalisation. There is only one situation where the conclusion is true and that is when it is a fact that all swans are indeed white. Argument two does not make a general claim. In a world where argument three has a true conclusion then so will argument two but argument two will have a true conclusion in many more situations. It could be argued, therefore, that argument two is more reliable than argument three and since the deductive argument is implicitly relying on argument three it would follow that the inductive argument (argument two) is more reliable than the deductive argument (argument one).

The only occasions when we can be confident that deductive arguments as a class are more reliable than inductive arguments is when the premises can be known to be true a priori, without relying on an associated implicit inductive argument:

Argument four
A pentagon has more sides than a square; a square has more sides than a triangle. Therefore, a pentagon has more sides than a triangle.