The Commens Dictionary

Quote from ‘Harvard Lectures on the Logic of Science. Lecture II, 1865’


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.

W 1:180
‘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
Jan 05, 2013, 10:54 by Sami Paavola
Last revised: 
Jan 07, 2014, 01:01 by Commens Admin