A real is anything that is not affected by men’s cognitions about it. An external object is anything that is not affected by any of man’s cognitions (whether about it or about anything else) to whom it is external. Exaggerate this, and you have the conception of what is not affected by any human cognitions. Take the converse of this definition, and you have the definition of the Ding an sich, as that which does not affect human cognition.
http://www.commens.org/dictionary/entry/quote-materials-monist-article-consequences-pragmaticism-vols-i-and-ii-r-3