Abductive Reasoning

Lillian had gathered some very pretty purple geranium flowers from a bush in the yard at the farm.  She was holding the flowers and noticed a purple petal on the table.

“Must be from the flower!” she said.

Background: The philosopher CS Pierce is well-known for insisting that in addition to the traditional categories of deductive reasoning and inductive reasoning, there is another category he called abductive reasoning.  The three-way distinction among these kinds of reasoning is now widely accepted and often illustrated as follows:

  • Deductive.  Suppose you have a bag of red marbles.  You draw one from the bag.  You can infer by deduction that it is red.
  • Inductive.  Suppose you have a bag of marbles of unknown colour.  You draw one from the bag.  You can infer by (weak) induction that the marbles in the bag are red. 
  • Abductive.  Suppose you have a bag of red marbles.  You notice a red marble lying near the bag.  You can infer by abduction that it is from the bag.

Since I had lectured at Melbourne Uni on this distinction, and used this way of illustrating it, a month or two ago, Lillian’s inference stood out as perfectly analogous to the classic illustration of the abductive inference.

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