Methodeutic is a term used by Charles Peirce to denote the third branch of logic. For Persian logic is a normative science, divided into three branches:
- Speculative grammar
- Critique
- Speculative rhetoric (Methodeutic).
The third branch delivers the theory of inquiry, which Peirce describes as “A search of the search – an inquiry in terms of the success of an inquiry (beyond gathering and examining the facts)” (CP 5.568n).