A weak mind does not accumulate force enough to hurt itself; stupidity often saves a man from going mad. We frequently see persons in insane hospitals, sent there in consequence of what are called religious mental disturbances.
 Oliver Wendell Holmes Sr., The Autocrat of the Breakfast-Table (1858). copy citation

Context

“—Insanity is often the logic of an accurate mind overtasked. Good mental machinery ought to break its own wheels and levers, if anything is thrust among them suddenly which tends to stop them or reverse their motion. A weak mind does not accumulate force enough to hurt itself; stupidity often saves a man from going mad. We frequently see persons in insane hospitals, sent there in consequence of what are called religious mental disturbances. I confess that I think better of them than of many who hold the same notions, and keep their wits and appear to enjoy life very well, outside of the asylums. Any decent person ought to go mad, if he really holds such or such opinions.” source