Lecture2: What is Life?

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?