Any proof using an instance of excluded middle or choice which is not constructively valid. For example, see the first proof on this page: http://www.cut-the-kn

Any proof using an instance of excluded middle or choice which is not constructively valid. For example, see the first proof on this page: http://www.cut-the-knot.org/do_you_know/irrat.shtml http://www.cut-the-knot.org/do_you_know/irrat.shtml