Lecture2: What is Life?

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A lecture at University of Melbourne in 2004

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Lecture2: What is Life?

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

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