module Foo

  def call_foo
    foo
  end

  def call_self_foo
    self.foo
  end

  def call_other_foo(other)
    other.foo
  end

  private # <-- change this to protected and see what changes

    def foo
      puts 'Foo#foo got called'
    end

end

class Bar

  include Foo

  def bar_call_foo
    foo
  end

  def bar_call_self_foo
    self.foo
  end

  def bar_call_other_foo(other)
    other.foo
  end

end

class NotFoo

  protected

    def foo
      puts 'NotFoo#foo got called'
    end

end

Bar.new.call_foo

Bar.new.call_self_foo

Bar.new.call_other_foo(Bar.new) rescue puts $!.message

Bar.new.call_other_foo(NotFoo.new) rescue puts $!.message