porque no puedo mandar llamar un metodo privado con self

gistfile1.txt

class A
  #defining static methods
  instance_eval do
    def x
      'x'
    end
    
    instance_eval do
      private
      def private_m
        'class private'
      end
    end
  end
  
  class_eval do
    public
    def public_m
      puts self.to_s
      self.private_m
    end
  end
  
  class_eval do
    private 
    def private_m
      'instance private'
    end
  end
  
  def to_s
    "instancia de A"
  end
  
end

puts A.x
puts A.send :private_m
puts A.new.public_m

puts A.instance_methods(false)

A.class_eval do
  def y
    'y instance'
  end
end

puts '---------'
puts A.instance_methods(false)
puts A.new.y

puts '---------'
puts A.class_eval('methods(false)')

A.instance_eval do
  def z
    'z result'
  end
end

puts '---------'
puts A.class_eval('methods(false)')

puts '---------'
puts A.z