Ruby on Rails

I still dont think I am fully clear on the Class class role.

For example:

ruby-1.8.7-p330 :010 > class Class
ruby-1.8.7-p330 :011?> def is_a?
ruby-1.8.7-p330 :012?> puts 'a'
ruby-1.8.7-p330 :013?> end
ruby-1.8.7-p330 :014?> end
=> nil
ruby-1.8.7-p330 :015 > A.is_a?
a
=> nil
ruby-1.8.7-p330 :016 > class A
ruby-1.8.7-p330 :017?> def a_is_a
ruby-1.8.7-p330 :018?> is_a?
ruby-1.8.7-p330 :019?> end
ruby-1.8.7-p330 :020?> end
=> nil
ruby-1.8.7-p330 :021 > @a = A.new
=> #<A:0x1069b8460>
ruby-1.8.7-p330 :022 > @a.a_is_a
ArgumentError: wrong number of arguments (0 for 1)

It says wrong number of arguments, despite there is nothing in argument
list in method definition. a_is_a is a getter method. It returns a value
returned by the class method is_a? defined in class Class. Since class A
is a class, it should have access to class methods of Class (and it
indeed works if you call the class method outside scope of instance
method definition and within the scope of the class A). Or is this an
issue of scope, where you cannot try to access a class method within an
instance method? But then why does it throw error it does?

Thanks for response.

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