can't affect mortals

A thumping sound made me turn my eyes back toward the house.
A man, burly, wearing a dark, hooded sweater and old jeans, was holding a long-handled crowbar in big, blocky hands. He shoved one hand into the space between the door and the frame, and with a practiced, powerful motion, popped the door from its frame and sent it swinging open.
Without an instant’s hesitation, Sir Stuart fired. So did the house’s spectral defenders. A hurricane of ghostly power hurtled down upon the man—and passed harmlessly through him. Hell, the guy looked like he hadn’t noticed anything at all.
“A mortal,” Sir Stuart breathed. He took a step forward, let out a sound of pain, and clutched at his side. His teeth were clenched, his jaw muscles standing out sharply. “Dresden,” he gasped. “I cannot stop a mortal man. There is nothing I can do.”


Ghost Story Chapter 5, Page 49