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A lecture at University of Melbourne in 2004
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What is Life?
Lecture 2
Problems of definition
• Hull’s Rule: For every rule in biology, there is an exception, or“Nothing is so absurd in biology that there is not at
least one example”
• A definition must cover all and only the things defined.
• But biology is fuzzy
Aristotle
• Prior to 1800, “biology” didn’t exist
• So Aristotle is not concerned just with “Life” but with all things - he makes no distinction
• What he wants to define is what makes things do what they do
• So he defines living things in terms of “soul”
Soul
Soul (psuche) means something like “motivating force” • Plants have only a “nutritive soul”• Animals also have a “sensitive soul” capable of
sensation• Hence they must have an “appetitive soul”, as do all
organisms capable of sensation, because they must have some desire
• Some animals have in addition a “locomotory soul”
Soul continued
• One of those, Man, alone also has the power of rational thought, or a “rational soul”
• Soul is the source of movement and growth, and it is the final cause of those faculties, that for which things are generated
• Life = potential to consume nutrition and to grow• Not identical with the body - a dualism• Soul is the actualisation of potential life
God All souls
Angels + perfection
Heaven + incorruption
Man + reason
Brutes + appetitive+ locomotory+ sensitive
Plants + nutritive
Flame + generation
Gems being
Great Chain of Being
Life?
Failings of Aristotle’s Psychism
• Explains little - basically a soul is that which moves, explaining why it moves; because it has soul
• Relies on “four causes” view of explanation
• Relies on “top-down” classification of things; aprioristic and non-empirical
Modern views
• Organic chemistry without life
• Vitalism, the doctrine that life has some special physical property, like a vital fluid or élan vital
• Vitalism died in the mid-20thC
Thermodynamic life - Schrödinger
• Before physical biology, life was something “vital”• Thermodynamic account of life - life feeds on
“negentropy”• Entropy is the tendency of systems to become
homogenous in their energy; that is, reach an energetic equilibrium
• Life requires a difference in energy levels - metabolic definition of life
Chemical life
• Originally by Haldane and Oparin• Miller-Urey experiment• Schopf’s CHON(SP) - Carbon, Hydrogen, Oxygen and
Nitrogen (plus Sulphur and Phosphorus)• Post Hoc definition - what about possible other
chemistries?• Still does not explain why some things live and others
don’t
Mathematical life - Information and Alife
• Von Neumann’s idea of “self-replicating robots”
• Turing’s idea of “reaction diffusion gradients” [but the whole horse is harder]
• Chaitin’s definition of life as a program producing information
• Artificial Life simulations - the “logical form” of all life.
Orgel’s combined definition
• Orgel’s CITROENS: – Complex– Information-producing– Objects, that– Evolve by– Natural Selection
Eigen - a physicist among the biologists
• Life is a dynamic state of matter
• It is organised
• Information is produced by natural selection
• It has a metabolism
• Eigen brings all the traditions together
Hypercycles
Dawkins and Hull - The Replicator
• William’s “evolutionary gene” becomes
• Dawkins’ Replicator– Longevity (over evolutionary time)– Fecundity (more made than can survive)– Fidelity (nearly perfect copying)
• Life begins at replication
So, where does that leave us?
• Is there a universal biology?– If so, does it need to be based on natural selection?
• Is there a universal chemistry of life?
• Are there necessary and sufficient conditions?
• Is “Life” a useful category?