The Commens Dictionary
Quote from ‘A Brief Intellectual Autobiography by Charles Sanders Peirce’
Thirdness is that mode or element of being whereby a subject is such as it is to a second and for a third; or rather, it is the characteristic ingredient of this definition, which is prominent in the ideas of instrument, organon, method, means, mediation, betweenness, representation, communication, community, composition, generality, regularity, continuity, totality, system, understanding, cognition, abstraction, etc. [—] Thirdness is subject to two grades of degeneracy. All genuine thirdness has a mental character.
This quote has been taken from Kenneth Laine Ketner's 1983 reconstruction of Peirce's 'Autobiography'