- See main article, Life, for perspective on the origin of these sundry topics.
Selected definitions of life
Marcello Bárbieri, Professor of Morphology and Embryology at the University of Ferrara, Italy, collected an extensive list of definitions of “Life” from scientists and philosophers of the 19th and 20th centuries. Those selected below resonate with the systems and thermodynamic perspectives of living systems:
- "The broadest and most complete definition of life will be "the continuous adjustment of internal to external relations". — Herbert Spencer (1884)
- "It is the particular manner of composition of the materials and processes, their spatial and temporal organisation which constitute what we call life." — A. Putter (1923)
- "A living organism is a system organised in a hierarchic order of many different parts, in which a great number of processes are so disposed that by means of their mutual relations, within wide limits with constant change of the materials and energies constituting the system, and also in spite of disturbances conditioned by external influences, the system ts generated or remains in the state characteristic of it, or these processes lead to the production of similar systems." — L. von Bertalanffy (1933)
- "Life seems to be an orderly and lawful behaviour of matter, not based exclusively on its tendency to go from order to disorder, but based partly on existing order that is kept up." — E. Schrodinger (1944)
- "Life is made of three basic elements: matter, energy and information. Any element in life that is not matter and energy can be reduced to information." — P. Fong (1973)
- "A living system is an open system that is self-replicating, self-regulating, and feeds on energy from the environment." — R. Sattler (1986)
Published collections of definitions of 'Life'
- Popa R (2004) Chronology of Definitions and Interpretations of Life. In: Popa R (ed.) Between Necessity and Probability: Searching for the Definition and Origin of Life. Berlin: Springer-Verlag 2004: pp 197-205 (Quotes and source-citations from 1885 to 2002)
- Barbieri M (2003) Appendix: Definitions of Life. In: The Organic Codes: An Introduction to Semantic Biology. Cambridge, UK: Cambridge University Press ISBN 0521824141 (Quotes from 1802 to 2002)
The exobiologists' view
Exobiologists and astrobiologists consider issues relating to the possible existence of extraterrestrial living systems. Dirk Schulze-Makuch and Louis Irwin attempted to distill the essential characteristics of a living system in their book Life in the Universe. They stress these characteristics, which resonate with the systems, thermodynamic and evolutionary perspectives discussed in the main article:
- a microenvironment, with a boundary between it and its external environment,
- the ability of that microenvironment to transform energy and matter from the environment to maintain a highly ordered, 'organizational' state (a low entropy state),
- therefore, the ability of that microenvironment to remain in thermodynamic disequilibrium with its environment,
- the ability of that microenvironment to encode and transmit information.
The gray zone
Not all entities that otherwise qualify as living reproduce themselves, although they exist as reproduced living things. Biologists call such living things sterile. Examples include programmed sterility (e.g., worker ants, mules); acquired sterility (due to acquired injury (disease) to the reproductive process; access sterility (lack of reproductive fitness); voluntary sterility (e.g., human couples). Obviously living things with the capacity to reproduce may die before reaching the reproductive stage in their life-cycle. Conversely, non-reproducing individuals may still effect reproduction of copies of their genes by facilitating the reproduction of kin, who share many genes (see kin selection).
Viruses would not qualify strictly as living things, but manage to 'reproduce' in living systems.
One might ask whether a spermatozoon qualifies as a living entity. From the thermodynamic perspective, one might answer affirmatively, as it keeps itself ‘living’ by doing cellular work. It has a compartmentalized internal organization functioning to keep it far-from-equilibrium. In that respect it resembles a motile bacterium. A spermatozoon reproduces, but in a different way than a motile bacterium: it does it through its parent’s progeny, which the spermatozoon plays an essential role in generating. It doesn’t have to hijack a cell’s machinery to reproduce; it cooperates with another cell (an ovum) to generate cells with machinery to reproduce it. Moreover, in reproducing that way, it subjects itself to meiotic crossover variation, just as its parent’s progeny does, contributing to the variation needed by natural selection to perpetuate the process of living on an earth with ever-changing environments.
Some modern views of the four Aristotelian causes of living things
According to University of Washington at Seattle Professor of Philosophy S. Marc Cohen’s reading of Aristotle, in describing the four causes of things, Aristotle asserts four different ‘kinds’ of causes that, combined, fully explain a thing. Scholars usually refer to them as the material, formal, efficient and final causes of things. But Cohen argues that Aristotle did not conceive of causes exclusively in the sense of cause-and-effect, which the word ‘cause’ connotes to us. He meant something like ‘things require four different kinds of explanations’, or ‘explaining things requires knowing four different kinds of explanatory factors that make them up’.
Only Aristotle’s ‘efficient cause’ — how a thing gets produced — appears to suggest cause-effect, implying that the material, formal and final ‘causes’ do not denote cause-to-effect. This section describes how some philosophers view Aristotle's 'causality' in relation to living things.
Substituting 'to make' for 'to cause'
To avoid confusing ‘explanation’ or ‘explanatory factor’ with ‘causation’ in the way we commonly think of it, Cohen suggests substituting a form of the more versatile verb ‘to make’ for ‘to cause’.
For a living organism, an apple tree, say, Cohen might translate Aristotle as saying:
- A particular collection of substances make up (constitute as elements) an apple tree — matter, parts, ‘material cause’.
- Having those substances organized in a particular way — we can readily see the organized form of a trunk, limbs, leaves, and apple-fruit — makes it (qualifies it as) an apple tree, an organized form — pattern, ‘formal cause’
- Seeds from an apple tree make (produce, cause to exist) an apple tree — producer, creator, organizer of the parts, ‘efficient cause’
- Providing us with apples as nutritious fruit makes it have (gives it) a purpose, or function — purpose, goal, function, ‘final cause’
Note that Cohen suggests four different ‘kinds’ of ‘make’, four different senses of the word, in keeping with Aristotle’s four different kinds of ‘causes’.
Note also, that together, those four kinds of ‘make explanations’ or ‘make explanatory factors’ give an explanation of an apple tree that, at one level of explanation, we can regard as complete:
- “What makes this an apple tree?” A particular collection of substances organized in a particular way by another apple tree that serves to supply us with apples to eat.
From that one can answer "what makes me a human?": A particular collection of substances organized in a particular way by activities of other humans who value their children. Of course we can say that of other animals too.
Reconciling Aristotle's four 'causes' and the 'explanation' of living things
Professor of Biochemistry, Jan-Hendrik Hofmeyr, at the University of Stellenbosch in South Africa, has elaborated on Professor Cohen’s interpretation of Aristotle’s four causes. The remaining discussion stems in part from Professor Hofmeyr’s scholarly exposition on cells as biochemical factories that autonomously fabricate themselves.
The first two of Aristotle’s four ‘explanations’, or ‘explanatory factors’ — what constituents make up an organism; what organized form makes it an apple tree — have received partial elucidation from the basic sciences of biochemistry, molecular biology, anatomy and physiology. Those speak to the parts and the organized parts forming a living thing. Note that the form of an organism must include its organizational state as well.
The third explanation — what makes the organized form — in modern biology, begins with cell theory — every cell from a cell — and expands to the idea that organisms produce organisms — self-fabrication, with growth and adaptability, and reproduction.
If organisms (organized forms) produce organisms (organized forms), then organized form produces organized form — self-fabrication — and the second and third explanations — formal and efficient causes — coincide.
The fourth explanation many scientists would claim exists outside of science — organisms have no purpose for those scientists.
However, the fourth explanation may coincide with the third, and thereby with the second. Professor Hofmeyr notes that some scientists argue that we know the purpose of an organism by what it does, that what it does in effect constitutes its purpose, its final cause. What it does, basically, we can view as operating to sustain its living state by continually reconstituting itself — continually re-fabricating its parts as they ‘wear out’ — continually adjusting the parts-fabrication process in response to internal and external stimuli or perturbations (homeostasis, adaptability), and fabricating more or less faithful, but living, copies of itself (reproduction).
- Note: In writing on living things, Aristotle himself recognized that final and formal causes coincided:
There are four causes underlying everything: first, the final cause, that for the sake of which a thing exists; secondly, the formal cause, the definition of its essence (and these two we may regard pretty much as one and the same); thirdly, the material; and fourthly, the moving principle or efficient cause.
If we view an organism’s ‘purpose’ to fabricate itself (in those several ways), then we can view its purpose — its ‘final cause’ — as coinciding with its ‘efficient cause’, the third explanatory factor for an organism. In other words, its purpose — defined as doing what it does — coincides with how it gets produced. Doing what it does (what makes it satisfy its purpose) gets it produced (makes the organism).
If a living thing’s ‘efficient cause’ coincides with its ‘formal cause’, as discussed above, then a living thing’s formal, efficient and final causes coincide. Thus the second, third and fourth explanatory factors for explaining an organism coincide. Doing what it does (‘final cause’) as an organized form (‘formal cause’) gets it produced as an organized form (‘efficient cause’).
But to fabricate itself a living thing must use the parts that constitute itself, which as we know, it largely produces for itself from externally acquired energy-rich building blocks (in the case of heterotrophs), or the combination of externally acquired free energy and simple materials (in the case of autotrophs) (see Life). The final cause, coinciding with the efficient and formal causes, thus explains the material cause. A living thing, doing what it does (‘final cause’) as an organized form (‘formal cause’) gets itself produced as an organized form (‘efficient cause’) from parts (‘material cause’) that it makes itself.
Thus living things do not need four different kinds of Aristotelian ‘causes’ or ‘explanations’ or ‘explanatory factors’. Indeed, it appears that the parts (material cause) assemble themselves spontaneously into the living thing — self-organization. Although a living system requires a flow of energy and matter through itself, characterizing it as an open system in thermodynamic terms, given that proviso it requires no independent material, formal, efficient and final cause, as the system, doing what it does (final cause), as an organized system, fabricates itself (efficient cause), as an organized system (formal cause), from parts (material cause) that it makes itself and that assemble themselves (efficient cause) into the living thing that does what it does (final cause).
Thus, according to the above analysis, a living system remains ‘closed’ within itself to Aristotelian causation in virtue of its nature as a self-fabricating and self-reconstructing factory. That cannot be said of non-living systems.
Citations and notes
- Bárbieri M. (2003) The Organic Codes; An Introduction to Semantic Biology. Cambridge: Cambridge University Press.
- Shapiro R. (2007). Speech at An Edge Special Event at Eastover Farm: Life: What a Concept!
- Note: Shapiro, professor emeritus of chemistry and senior research scientist at New York University, stated: The term exobiology was coined by Joshua Lederberg in the early 1960s to describe any life forms found outside of Earth. In the 1990s it got resurrected at astrobiology, which defined itself as the science that encompasses all life, past, present, and future. Which swallows all of biology and then burps — and if anything exo is ever found, it will swallow that too. So it can't be accused, as exobiology once was, of being the only subject that had no subject matter to study.
- Billings L, Cameron V, Claire M, Dick GJ, Domagal-Goldman SD, Javaux EJ, Johnson OJ, Laws C, Race MS, Rask J, Rummel JD, Schelble RT, Vance S. (2006) The astrobiology primer: an outline of general knowledge--version 1, 2006. Astrobiology 6:735-813 Full-Text Also Here
- Outstanding reference on the principles of life and its possible existence beyond Earth (79 pages)
- Schulze-Makuch D, Irwin LN, Definition of Life. In Life in the Universe. Berlin: Springer-Verlag 2004: Chapter 2. pp 8-34 Link to Summary and Full-Text
- Cohen SM. (2006) The four causes.
- Hofmeyr JH. (2007) The biochemical factory that autonomously fabricates itself: A systems biological view of the living cell. In: Boogerd FC, Bruggeman FJ, Hofmeyr JH, Westerhoff HV (editors). Systems Biology: Philosophical Foundations. Elsevier, Amsterdam. ISBN 978-0-444-52085-2
- Aristotle. (2007) On the Generation of Animals. Translated by Arthur Platt. eBooks@Adelaide