For a while, this problem was intermittent. Clone, Healing, History, Paint, Eraser, and Dodging brushes spontaneously (and not in response to any apparent action) stopped working, but after closing the program and restarting the computer, became functional again. Now, they've stopped working and neither closing and reopening Photoshop nor restarting the computer solves the problem. I've deleted my preferences file, but that hasn't brought the brushes back to life either. Adobe tech support (which in the past has been responsive and helpful but now seems nearly worthless) referred me to a download called "Clean Script CS4," but I'm hesitant to try this for two reasons: 1.) it is represented on the Adobe website as a solution to installation problems (and my problem isn't with installation), and 2.) it entails uninstalling and reinstalling Photoshop. Does anyone have insights into the problem of inoperative brushes?