Topic 22: Abductive Argument

You are walking in the park and come across a newborn chick on the ground. You look up and see a nest with a large bird glowering warily down. You can also hear chirping from the direction of the nest. You conclude, naturally, that the newborn chick had fallen from the nest.

This is abductive reasoning. In essence, it is reasoning from a body of evidence to some situation or ‘hypothesis’ that would explain the evidence. In the current example, you did not actually see the chick fall out of the nest, but you infer it probably did, because that would explain why there is a vulnerable newborn chick on the ground underneath a nest containing a mother bird and other chicks.

Figure 2.24

Abductive reasoning is inferring that some hypothesis is true because

it would causally explain some body of evidence.

The term ‘abductive’ is derived from the Latin ab (from) and duco (lead), conveying the idea that abductive reasoning works ‘back’ or away from some phenomenon to something prior which was responsible for it.

In its simplest form, abductive reasoning considers just one hypothesis, and assumes that the hypothesis either does or does not explain the evidence. Sometimes, multiple hypotheses are entertained (e.g., chick fell out of nest; chick was thrown out of nest; chick was placed on the ground by a researcher; etc.) and the most plausible of these hypotheses is taken to be true. In this more elaborate form, abductive reasoning is also known as ‘inference to the best explanation’.

Abductive reasoning is very common. It is the dominant form of reasoning in areas such

as:

- Medical diagnosis, in which a doctor tries to identify the source of the various symptoms displayed by a patient.

- Science, which has a major goal describing the causal structure of the world, and in which scientists are constantly inferring to the truth of causal propositions (e.g., that AIDS is caused by HIV infection).

- Problem solving, in which we attempt to determine the underlying cause of some problem, such as a car failing to start.

Abductive reasoning has a striking similarity, in basic shape, to the argument pattern known as ‘Affirming the Consequent’. This has the structure ‘ If P then Q; Q therefore P’; for example, if it is raining, then it is wet outside; it is wet outside; therefore it is raining. Affirming the Consequent is a well-known fallacy of deductive logic; it has the superficial appearance of a deductively valid argument, but the premises do not in fact guarantee the contention (e.g., it may be wet outside due to melting snow).

However, abductive reasoning is not intrinsically fallacious. Abductive arguments are intended not to ‘guarantee’ their contentions, but to render them more probable. The degree to which they do this – i.e., their strength – depends on the reliability of the evidence, the plausibility of the hypothesis as an explanation of the evidence, and the relative plausibility of alternative hypotheses. Conclusions are drawn tentatively or provisionally, since it is always possible that the body of evidence may change or another, more plausible hypothesis may arise.