​The Boundary of the Living: Minimal Forms of Differentiating

Autopoiesis, defined as the capacity of a differentiating structure to sustain differentiation, constitutes the first ontological condition for life. However, this definition raises a further question: what is the minimal form at which differentiation can be considered alive? In other words, where is the lower boundary of life, if life is understood as a stable differentiator that differentiates itself?

To answer this, we must first establish: not all stability of differentiation constitutes life, and not every reproduction of structure implies a differentiating retention of boundary. There exist forms that are stable and even reproducible but do not differentiate. These are pre-living configurations in which differentiation is stabilized but not centered—that is, not retained from within through a differentiating node.

An example of such a limiting case is the virus. A virus is capable of reproduction only within an already living system. Outside the cell, it is inert: it possesses no mechanism for retaining differentiation, does not restore a boundary between itself and the external. Its structure transmits differentiation but does not differentiate—it is copied, but does not maintain itself as a differentiator. In this sense, a virus is not alive, though it embeds itself into a differentiating coupling by exploiting another's capacity to differentiate.

Thus, the virus marks a boundary of life: it shows that life does not begin with replication, but with the differentiating retention of boundary, however minimal. The virus may be called a pre-living form in which differentiation is already structured but not retained as an act of differentiation. It depends on what is already alive for the continuation of its difference.

On the other hand, structures such as crystals retain the form of difference with high regularity but do not regulate their retention. Their reproduction is mechanical, determined by external conditions. Prions—pathologically active proteins—alter other structures but do not differentiate or undergo differentiation in their own act. These forms are stabilized but not active in a differentiating sense. Their stability results not from internal retention, but from the coupling of conditions.

In contrast to these pre-living forms, minimal living systems—such as the simplest autopoietic cells (e.g., synthetic minimal cells created in artificial biology experiments)—demonstrate a basic level of differentiating retention. These cells, composed of only a minimal set of molecules (lipids for membranes, DNA or RNA as code, enzymes for replication), are capable of maintaining a boundary between themselves and the external environment, reproducing themselves, and regulating internal processes. They differentiate themselves as differentiators, maintaining a minimal boundary through cyclic metabolic processes—something that distinguishes them from viruses and prions.

Thus, we may outline three zones:

Below life: forms that preserve differentiation without differentiating (e.g., crystals, prions).

At the boundary of life: forms in which differentiation is transmitted but not retained from within (e.g., viruses).

Living: forms in which differentiation is retained, reproduced, and differentiated as one’s own differentiation.

Life begins where the differentiating not only differentiates, but retains itself as the center of differentiation. This differentiator is capable of repeating differentiation while preserving a boundary—even if that boundary is minimal and plastic. The threshold of life, in this sense, is not absolute, but it is ontologically distinct.

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