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Types of reasoning: deduction, induction, abduction

Before we look at individual argument patterns, it is worth taking a look at the basic forms of inference. Charles S. Peirce distinguished three of them, which differ in what we presuppose and what we want to gain: a rule, an individual case or an explanation.

Deduction: from the rule to the case​

In deduction we infer from a general rule to an individual case. If the premises are true, the conclusion is necessarily true: deduction is truth-preserving, but creates no new knowledge about the world; it only makes explicit what is already contained in the premises.

  1. Rule: All humans are mortal.
  2. Case: Socrates is a human.
  3. Result: Therefore Socrates is mortal.

Induction: from the case to the rule​

In induction we generalize from observed individual cases to a rule. The inference is not compelling, but only more or less probable: a single exception can overturn the rule. In return, induction delivers new knowledge that goes beyond the observations.

  1. Case: Socrates, Plato, Aristotle … have died.
  2. Result (rule): Therefore presumably all humans are mortal.

Abduction: from the finding to the best explanation​

In abduction we seek the most plausible explanation (hypothesis) for a surprising observation. It too is not compelling, but a justified conjecture that can later be confirmed or refuted.

  1. Finding: The lawn is wet.
  2. Rule: If it has rained, the lawn is wet.
  3. Best explanation: Presumably it has rained.

How they interact​

The three forms interlock. Deduction lives on universal statements ("All A are B"), yet, strictly speaking, we can never prove such universal sentences by observation. We mostly obtain them inductively, by generalizing from many individual cases. Deduction is therefore only as certain as the inductively acquired rules on which it is built: it inherits its rigor from premises that are themselves only probable. Abduction, in turn, generates the hypotheses that we subsequently test inductively and use further deductively.

Key point

Deduction secures, induction generalizes, abduction explains. Only deduction is truth-preserving; induction and abduction extend our knowledge, but remain uncertain.

Further reading: Abduktion, Induktion, Deduktion (arbeitsblaetter.stangl-taller.at)