From: no_spam_anglewyrm@hotmail.com   
      
   "jonathan"  wrote in message   
   news:417b2dad$1@news.unimelb.edu.au...   
   > If 2 variables correlate we can't imply causality. It could be that   
   > the 1st variable cause the other. It could be the other way around. It   
   > could be that they both correlates with a third variable.   
   >   
   > I've heard that if we know 3 variables, then we can imply causality.   
   > How in the earth the 3rd variable help us that?   
      
   What is causality? To my mind, the words causality and cause describe an event   
   between objects that changes something. I painted a blue marble red. It was not   
   the paint on my brush that caused the blue marble to be a red marble; that   
   paint   
   is a noun. Nor was it I that caused it, for I too am a noun. It was the act of   
   me painting the blue marble with red paint that was the cause.   
      
   To express this numerically, I would guess we are talking about a change, some   
   delta over time. Is a dependant relationship the same as a causal relationship?   
   What of this function:   
      
   paint ( blue_marble, red_paint ) = red_marble   
      
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