Where are methods defined at the ruby top level?

At the top level, method definition should result in private methods on Object, and tests seem to bear this out:

def hello; "hello world"; end

Object.private_instance_methods.include?(:hello) #=> true
Object.new.send(:hello) #=> "hello world"

However, the following also works at top level (self.meta is the eigenclass of main):

self.meta.private_instance_methods(false).include?(:hello) #=> true

It appears that the hello method is simultaneously defined on the eigenclass of main as well as on Object. What's going on? Note that the false parameter to private_instance_methods excludes super class methods from the method list.

First of all, this behavior and the underlying reasoning have always existed; it's nothing new to 1.9. The technical reason it happens is because main is special and treated differently than any other object. There's no fancy explanation available: it behaves that way because it was designed that way.

Okay, but why? What's the reasoning for main to be magical? Because Ruby's designer Yukihiro Matsumoto thinks it makes the language better to have this behavior:

Is so, why are top-level methods not made singleton-methods on this object, instead of being pulled in as instance methods on the Object class itself (and hence into all other classes i.e. more namespace pollution than is usually intended). This would still allow top-level methods to call other top-level methods. And if the top-level object were referred to by some constant like Main, then those methods could be called from anywhere with Main.method(...).

Do you really wish to type "Main.print" everywhere?

Further on in the discussion, he explains that it behaves this way because he feels the "assumption is natural."

EDIT:

In response to your comment, your question is aimed at why main's eigenclass seems to report hello as a private instance method. The catch is that none of the top-level functions are actually added to main, but directly to Object. When working with eigenclasses, the instance_methods family of functions always behave as if the eigenclass is still the original class. That is, methods defined in the class are treated as being defined directly in the eigenclass. For example:

class Object
  private
  def foo
    "foo"
  end
end

self.send :foo  # => "foo"
Object.private_instance_methods(false).include? :foo  # => true
self.meta.private_instance_methods(false).include? :foo  # => true

class Bar
  private
  def bar
    "bar"
  end
end

bar = Bar.new
bar.send :bar  # => "bar"
Bar.private_instance_methods(false).include? :bar  # => true
bar.meta.private_instance_methods(false).include? :bar  # => true

We can add a method directly to main's eigenclass, though. Compare your original example with this:

def self.hello; "hello world"; end

Object.instance_methods.include? :hello  # => false
self.meta.instance_methods.include? :hello  # => true

Okay, but what if we really want to know that a given function is defined on the eigenclass, not the original class?

def foo; "foo"; end  #Remember, this defines it in Object, not on main
def self.bar; "bar"; end  #This is defined on main, not Object

foo  # => "foo"
bar  # => "bar"

self.singleton_methods.include? :foo  # => false
self.singleton_methods.include? :bar  # => true