The principle of calling things that are not as though they were is the spiritual principle through which everything physical becomes manifest.
God created the light by calling for "light" when only darkness was there. Jesus used this same method, calling the lepers clean, and the dead to life, and peace to the storm. You must call for what you desire. if you want your dog to come, you call the dog, not the cat. You call for what is not there. Whatever you call in the natural will come. Call what does not exist and continue to call until it manifests.