I have always thought that there are multiple dimensions linked to our world and mirrors were one of the doorways to one or more different dimensions.

I have another theory about a dimension(s) that can be reached from our world through dreams. That those who people say have large imaginations are the people who have probably been to this dimension more times than those who are uptight and lack creativity. In this dimension(s) lives all the creatures from ancient lore that people nowadays say don't exist. For example mermaids, they have been seen in different countries but have been given different names. Dragons are an even better example. China has been using dragons for centuries. The scots have their lochness monster(which i do believe travels between the dimensions freely). There is another lochness monster creature in NY called Champy in Lake Champlane. So perhaps these dimensions, either mirror or dream, do exist and they are just something for few to believe and others to critisize.