This is a key point in tantra.  It comes from the recognition that Shiva is being all.  Everything in the world and even everything in your mind is Shiva in disguise. So, what part of the world should you reject?  What part of you should you reject?  Certainly, your mind needs some conditioning, like a runner preparing for a marathon.  Your mind needs… 

—  Gurudevi Nirmalananda

From Gurudevi’s full discourse “Celebrating Spiritual Greatness“