You can't have an explicit receiver for private methods. See below.

class Foo
  def try1
    something_private()
  end

  def try2
    self.something_private()
  end

  private
  def something_private()
    puts "I'm doing private things!"
  end
end

f = Foo.new
f.try1
#=> I'm doing private things!

f.try2
#=> tmp.rb:7:in `try2': private method `something_private' called for
#<Foo:0x282dd70> (NoMethodError)
#=>  from tmp.rb:19

f.something_private
#=> tmp.rb:21: private method `something_private' called for
#<Foo:0x282ddd4> (NoMethodError)