[Rails] ruby and rails wont work even they're already installed

[Mongskie]

mongskie@mongskie-os:~$ gem
/usr/bin/gem:4: undefined method `manage_gems' for Gem:Module
(NoMethodError)


mongskie@mongskie-os:~$ rails
/usr/lib/ruby/1.8/active_support/dependencies.rb:55: uninitialized
constant ActiveSupport::Dependencies::Mutex (NameError)
        from /usr/local/lib/site_ruby/1.8/rubygems/custom_require.rb:36:in
`gem_original_require'
        from /usr/local/lib/site_ruby/1.8/rubygems/custom_require.rb:36:in
`require'
        from /usr/lib/ruby/1.8/active_support.rb:56
        from /usr/share/rails-ruby1.8/railties/bin/../lib/rails_generator.rb:
28:in `require'
        from /usr/share/rails-ruby1.8/railties/bin/../lib/rails_generator.rb:
28
        from /usr/share/rails-ruby1.8/railties/bin/rails:14:in `require'
        from /usr/share/rails-ruby1.8/railties/bin/rails:14

I always get these messages. Why is this. Pls help me.

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