Flowing boundaries in autopoietic systems and microniche construction.

Organismal boundaries might seem like a straightforward and unproblematic organismal feature to study. They serve as fundamental demarcation lines that differentiate life from its environment, define identity, and maintain the functionality of organisms. But do they amount to an actual demarcation of organismal self? In this paper, we examine the philosophical and biological underpinnings of these boundaries, explore the essentialist and non-essentialist perspectives, and categorise organismal boundaries into three types: life-defining, physical, and those based on structural coupling. We shall argue largely against excessive reliance on physical boundaries, point to the inconsistencies and limitations of such thinking with the help of some formal approaches to boundaries (e.g., Markov blankets or theories such as (M, R) systems or the theory of autopoiesis), and try to harmonise the approaches by introducing a concept of boundary based on structural coupling. Autopoietic systems, such as cells, are structurally coupled to their environment, meaning their structures and those of their environment constantly influence each other. Organisms exhibit varying levels of the coupling capacity, of extending beyond their membranes to modify environments on scales ranging from molecular to planetary. Unicellular organisms, colonies, and multicellular entities construct niches that shape their survival and evolution. Building on the niche construction theory, we introduce the concept of microniches to describe various controlled spaces within organisms whose status of 'internal' is not always straightforward from the host perspective (e.g., intercellular spaces, digestive systems, or xylem). In the next step, we explain how these microniches are a direct result of structural coupling and how this concept can explain what is or is not part of a biological entity. We conclude with a discussion of Kantian organic wholes, starting with the cell in its entirety enclosed by a membrane and moving on to higher-order structures such as multicellular organisms or colonies, which differ in how they are established. Organic wholes of various levels are defined by informational boundaries and shared evolutionary norms that enable cohesion, cooperation, and distinction from the external environment across diverse biological and cultural systems. By integrating various philosophical and biological perspectives, we want to deepen our understanding of how life defines and sustains its boundaries and challenge certain established forms of thinking about organismal boundaries, which often rely on the physical or spatial approach.