Quote from ACIM Text Chapter 2, Section 8 - Cause and Effect.

I cannot let you leave your mind unguarded, or you will not be able to help me. Miracle working entails a full realization of the power of thought in order to avoid miscreation. Otherwise a miracle will be necessary to set the mind itself straight, a circular process that would not foster the time collapse for which the miracle was intended. The miracle worker must have genuine respect for true cause and effect as a necessary condition for the miracle to occur.