jeudi 15 janvier 2009

「生命を定義する」 Définir la vie (4)

4 février 2008

Jacques Reisse "Why define life?"

物理学者はエネルギーの定義を持っていないが (ファインマン)、法則はある。

definition of mass in physics : highly concentrated form of energy
definition of time ?

R. Feynman : time, mass という定義が不明瞭なものから、物理法則は作ることができる。

biology is not the study of life, but of living system.
 Life = shared properties in all living systems
Why define life? --- biologists know living systems; not necessary to know life
● first living organisms?
● extraterrestrial life ?
● man-made system ?

Ponnamperuma, Cyril (1923-1995)
  "life must be common in space"

Aristotle : impossible to draw line between living and non-living, in broken sequence from lifeless to animals

Systems can be partially living
● true at the beginning of life
● still true today (virus, spore)
● analogy betw non-living & living

axioms of aristotelian logic
1) law of noncontradiction
   A cannot be both B & non-B
2) law of excluded middle
   A must either B or non-B
3) law of identity
   A will always be A

Lotfi Asker Zadeh (1921-)
1965 : Fuzzy Set:
1973 : Fuzzy Logic
multivalued logic
   A is not necessarily B or non-B but could be caracterized by a value between 1 (set B) and 0 (set non-B). Law of excluded middle is rejected.

Not all or none.
  robot : partially living (0.2 living)
  virus : 0.7 living

● better not to search for definition of life

 definition should be done in a context
 Freud : concept があれば、definition がなくとも前に進む?
 life in relation to entropy : low entropy but open system
 definition がなければ、other life forms を探せない
 物理学者はenergy, mass, timeの定義ができていない。生物学者が生命の定義ができていなくてもよいのではないか?

Gaia hypothesis - an ecological hypothesis that proposes that living and nonliving parts of the earth are viewed as a complex interacting system that can be thought of as a single organism

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