---
title: "Definition of life"
chunk: 2/3
source: "https://en.wikipedia.org/wiki/Definition_of_life"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T04:17:39.577696+00:00"
instance: "kb-cron"
---

A problem with defining life by a number of characteristics is that it can provide false positives. Long and detailed lists leave potential lifeforms out, and small lists may include things that were not intended to be considered alive. For example, Crystals can grow and tend towards equilibrium, similar to homeostasis, but are not alive. Robert Saphiro and Gerald Feinberg proposed that life is the activity of a biosphere, defining biosphere as "a highly ordered system of matter and energy characterized by complex cycles that maintain or gradually increase the order of the system through the exchange of energy with the environment"; a definition that may be too broad. There are four possible ways to organize a definition. The first one is that there are a number of features and all of them must apply for something to be alive; if something has only some features but not others, then it is not. This is the method used by divulgation outlets. The second is that there is a single necessary and sufficient condition that can define the presence or absence of life. The third is that there are several necessary and sufficient conditions that define life; this is the one used in science. And finally, there may be several types of life without a common characteristic between them all.
Dr. Carol Cleland, a member of the NASA Astrobiology Institute, considers that the problem is caused by the vagueness of spoken language, and that science does not need a definition of life, but rather a general theory of living systems. She compares the problems defining life with the problems defining substances in the Middle Ages, before the discovery of molecules, and points out that nitric acid was considered a type of water back then because it shared some superficial properties. However, a general theory can not be formulated before a sample of extraterrestrial life can be found and studied. At this point, there is not enough data to formulate such a theory, as it is unknown if life is abundant in the universe or just a rarity exclusive of Earth.
Life on Earth is carbon-based life, and uses water as a solvent. It is often assumed that life in other planets may have a similar composition, disregarding hypothetical types of biochemistry. Scientist Carl Sagan defined it as "carbon chauvinism". However, as those lifeforms are only theoretical, the details of their metabolism are unknown and it would be complex to define what to seek when seeking such lifeforms. However, although it is accepted that life can be composed of substances other than carbon and water, their properties still make them the better ones suited for it.
The NASA defines life as a "self-sustaining chemical reaction capable of Darwinian evolution". Adaptation works with natural selection, but it is unclear if human beings are still subject to it. In nature unfit creatures would not reproduce and would not pass their genes to later beings, ensuring that only the best individuals did so, but human beings are capable of compassion and to practice medicine, which may negate the process. And it's hard to test a being and detect if it's capable of evolution, as evolution takes place in the species over time and not in specific individuals. It is also unclear if Darwinian evolution is a feature of all life, or just a characteristic of life on Earth. However, no credible alternative to it has been formulated. Evolution can also find false-positives such as computer virus coded with evolutionary algorithms. Evolution does not explain either the origin of life, which is understood to be the result of abiogenesis, a chemical evolution rather than a biological one. It is still unclear if it would be possible to recognize the first organism capable of evolution: some scientists think that had to be a clear first living organism, and others that it would be a slow process with several stages.

=== Viruses ===
Viruses pose a challenge to the formulation of a definition of life, because of their unique system of reproduction. They are incapable of reproduction on their own, but they can reproduce by infecting a living cell and exploiting its reproduction for their own benefit. As a result, they are considered a borderline case, in the frontier between the living and non-living. Prions, abnormal protein molecules, are another borderline case, as they are also incapable of reproduction, but can infect other proteins and turn them into prions. Meaning, they don't make copies of themselves, but they increase their numbers by turning others. They are usually considered non-living, although they present some ambiguity.

== History ==

=== Aristotle ===

Aristotle emphasized that theory should be based on empirical investigations. He studied animal life by observation, experimentation, and dissection. He distinguished living things by their ability to use food, reproduce, perceive, move, and think. He compared the structure and actions of a living thing to the blueprints and tools used to build a house, as everything is prearranged to reach a desired end state. He noted that a house does not get built or maintained by itself and requires actual planning and work by humans, whereas living organisms build and regenerate themselves. This was difficult to reconcile with a mechanistic conception of causation, so he postulated a special form of natural causation, the "soul", that allows living things to be self-causing. He posited four types of causation: efficient, material, formal, and final. The concept of a soul allows animals to have final causations without an external guiding hand.