Applying an evolutionary perspective to Peirce’s work on signs, Deacon used iconic, indexical, and symbolic reference to describe “the logic of thought processes,” specifically looking at how they inform the development of language abilities. In doing so, he also outlined an emergent systems perspective of the relationships among these types of reference. This outline provides a basis for understanding cognition as an emergent system of perceptual, associative, and conceptual representation.
Probably the most successful classification of representational relationships was … provided by the American philosopher Charles Sanders Peirce … He distinguished three categories of referential associations: icon, index, and symhol … describing the nature of the formal relationship between the characteristics of the sign token and those of the physical object represented …
As a first approximation these are as follows: icons are mediated by a similarity between sign and object, indices are mediated by some physical or temporal connection between sign and object, and symbols are mediated by some formal or merely agreed-upon link irrespective of any physical characteristics of either sign or object …
The great philosophers of mind … in one way or another argued that these three modes of relationship describe the fundamental forms by which ideas can come to be associated. Peirce took these insights and rephrased the problem … in terms of communication, essentially arguing that all forms of thought (ideas) are … organized by an underlying logic … that is not fundamentally different for communication processes inside or outside of brains. If so, it might be possible to investigate the logic of thought processes by studying the sign production and interpretation processes in more overt communication.
The Symbolic Species: Chapter 3 – Symbols aren’t simple | Deacon
Peirce’s Theory of Signs | Stanford Encyclopedia of Philosophy
Symbol, Index, Icon | Theories of Media Glossary (University of Chicago)
Semiotic elements and classes of signs | Wikipedia