Why does defining a method at the top level affect other user-defined classes?

If I define a top level method like this:

def inspect(x)
  # do something useful...
end

Then calling #inspect on a user-defined class stops working:

class Foo; end
p Foo.new # `inspect': wrong number of arguments (0 for 1) (ArgumentError)

However, it keeps working for classes like NilClass and String:

p nil    # prints 'nil'
p "test" # prints '"test"'

I thought that one explanation for this behaviour could be that top-level execution may be in the Object class itself, but it turns out that it's in an instance of Object called main. Doesn't this 开发者_如何学Gomean that methods defined here shouldn't affect other classes?

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main is a special place. Any methods defined there are defined as private instance methods of Object. This is so you can define pseudo-functions that can be called in any context without an explicit receiver.