This paper proposes a new paradigm for defining life: an entity with cognition and need. This definition transcends traditional approaches based on metabolism, reproduction, and cellular structure by identifying life through functional criteria rather than material substrate.NASA's widely adopted definition—"a self-sustaining chemical system capable of Darwinian evolution"—faces critical limitations in providing clear boundaries. While it correctly excludes viruses (which lack metabolism), the metabolic criterion alone does not fully explain why certain complex entities lack life. Our framework provides clearer reasoning: life requires both cognition (adaptive information processing through active engagement) and need (autonomous goal-directedness).Building on autopoiesis theory (Maturana & Varela) and cognitive agency theory (Thompson, Lyon, Godfrey-Smith), we argue that cognition and need are the two fundamental dimensions of life's existence. Cognition—defined as a system's capacity to modulate its internal states based on sensory inputs and/or past experiences, thereby engaging in adaptive interactions with its environment (Varela, Thompson, & Rosch, 1991)—represents life's adaptive information processing. Need—defined as any environmental condition or resource toward which an autonomous system displays consistent approach or avoidance responses, significantly affecting the system's functional integrity—represents life's intrinsic motivation. Our framework demonstrates their co-evolution: need drives cognitive evolution, while enhanced cognition enables more complex needs.We classify life into seven grades from instinctive life (bacteria, archaea) to higher-order life (theoretical concept), providing a hierarchical framework that accommodates life's vast diversity. This classification is explicitly open-ended, acknowledging that future discoveries may reveal life forms beyond our current understanding. Importantly, viruses are excluded from this classification as they lack both genuine cognition (exhibiting only chemical specificity) and autonomous need (exhibiting only physical-chemical processes).This definition offers new strategies for extraterrestrial life detection based on cognition signatures (active exploratory movements, adaptive responses, sensorimotor coupling) and need indicators (autonomous goal-directed behavior, resource-seeking patterns). This approach does not depend on specific chemical foundations and can identify life in any form, addressing a critical limitation of current astrobiology methods.
Heng Liu (Sun,) studied this question.