Why Do Mystics Talk About “Purgation” or “Purification”?

A reader of this blog wrote to me:

I recently attended your workshop on Christian mysticism. We talk about purification as part of the contemplative path. I am wondering — Why is this important to practicing contemplation and how do you do it? Is it a matter of right belief/thought — orthodoxy — or is it right practice — orthopraxis — or both. Why is thi…