Why do scientist have different opinions regarding the existence of aliens?

Correspondence with the editor of the scientific magazine "Bild der Wissenschaft" Rüdiger Vaas
Abridged version as letter to the editor in "Bild der Wissenschaft" 6/2002, p.18
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Intelligent Life in the Cosmos –
Facts, Presumptions, Refutations

Published in
Human Beings in the Cosmos III
Peter R. Sahm (Ed.), Shaker Verlag GmbH, 2002
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There is nobody out there!

Abridged version published with the title
"Da draußen ist keiner!"
in »Bild der Wissenschaft«, issue May 2000, p.62-63
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Brothers in Space? The Probability of Intelligent Life

Prof. Dr. Ulrich Walter
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Welcome on Planet Earth ...?

Prof. Dr. Ulrich Walter
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What do Aliens look like?

Prof. Dr. Ulrich Walter
in: Star Observer Spezial "Sind wir allein im All?", March 1999, p.71-79
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