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Digital Companion to C. S. Peirce
‘À Posteriori Reasoning’ (pub. 05.01.13-10:54). Quote in M. Bergman & S. Paavola (Eds.), The Commens Dictionary: Peirce's Terms in His Own Words. New Edition. Retrieved from http://www.commens.org/dictionary/entry/quote-harvard-lectures-logic-science-lecture-ii-1865.
Term: 
À Posteriori Reasoning
Quote: 

There is a large class of reasonings which are neither deductive nor inductive. I mean the inference of a cause from its effect or reasoning to a physical hypothesis. I call this reasoning à posteriori. If I reason that certain conduct is wise because it has a character which belongs only to wise things, I reason à priori. If I think it is wise because it once turned out to be wise, that is if I infer that is is wise on this occasion because it was wise on that occasion, I reason inductively. But if I think it is wise because a wise man does it, I then make the pure hypothesis that he does it because he is wise, and I reason à posteriori. The form this reasoning assumes, is that of an inference of a minor premiss in any of the figures. The following is an example.

Light gives certain fringes         | Ether waves give certain fringes
Ether waves give these fringes | Light is ether waves
.: Light is ether waves               | .: Light gives these fringes.

[—]

The difference in their general character between the three kinds of reasoning is strongly marked. A consequent is inferred à priori, an antecedent à posteriori, and the nexus between them inductively.

Source: 
Peirce, C. S. (1865). Harvard Lectures on the Logic of Science. Lecture II, 1865. MS [W] 95;MS [R] 341, 765.
References: 
W 1:180
Date of Quote: 
1865
URL: 

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