Complementarity is an epistemological principle derived from the subject – object or observer – system dichotomy, where each side requires a separate mode of description that is formally incompatible with and irreducible to the other, and where one mode of description alone does not provide comprehensive explanatory power. The classical physics paradigm, on which biological, social and psychological sciences are modelled, completely suppresses the observer or subject side of this dichotomy in order to claim unity and consistency in theory and objectivity in experimental observations. Quantum mechanical measurements have shown this paradigm to be untenable. Explanation of events requires both an objective, causal representation and a subjective, prescriptive representation that are complementary. The concepts of description and function in biological systems, and goals and policies in social systems, are found to have the same epistemological basis as the concept of measurement in physics. The concepts of rate-dependent and rate-independent processes are proposed as a necessary distinction for applying the principle of complementarity to explanations of physical, biological and social systems.

All sciences have epistemic assumptions, a language for expressing their theories or models, and symbols that reference observables that can be measured. In most sciences the languages in which their models are expressed are not the focus of their attention, although the choice of language is often crucial for the model. On the contrary, biosemiotics, by definition, cannot escape focusing on the symbol-matter relationship. Symbol systems first controlled material construction at the origin of life. At this molecular level it is only in the context of open-ended evolvability that symbol-matter systems and their functions can be objectively defined. Symbols are energy-degenerate structures not determined by laws that act locally as special boundary conditions or constraints on law-based energy-dependent matter in living systems. While this partial description holds for all symbol systems, cultural languages are much too complex to be adequately described only at the molecular level. Genetic language and cultural languages have common basic requirements, but there are many significant differences in their structures and functions. Relevance: The paper expresses the classical epistemological mind-matter problem at the simplest evolutionary level, which begins with self-replication. At this level I call it the symbol-matter problem, and I discuss the physical and epistemic conditions for symbol systems and languages to arise.

In this dialogue, we discuss the contrast between inexorable physical laws and the semiotic freedom of life. We agree that material and symbolic structures require complementary descriptions, as do the many hierarchical levels of their organizations. We try to clarify our concepts of laws, constraints, rules, symbols, memory, interpreters, and semiotic control. We briefly describe our different personal backgrounds that led us to a biosemiotic approach, and we speculate on the future directions of biosemiotics.