In practice, two perfect substitutes are two goods or services that can replace each other entirely giving the comsumer exactly the same benefit using anyone of them.
Example Usage of substitute
heather_huntley: There is absolutely no substitute for a genuine lack of preparation.
madtownbabe: @jlkoctober It's like a white russian, only you substitute Bailey's for the cream. Totally yummy - and lethal!!!
kodelas: Development can never be a substitute for self respect, development can never heal the wound of huilimiation or suppression..!
