Police departments issue their officers Kevlar vests to stop bullets, and thick helmets and even shields to protect them from bottles and bricks. But there is nothing in the equipment room to give an officer thicker skin.

That tool  as vital to an officer’s safety and the public’s as anything clipped to his belt  is developed in training, and its strength differs from one officer to the next.

The issue of tolerance, in fact, lies at the heart of the dispute surrounding the arrest of the Harvard professor Henry Louis Gates Jr. in Cambridge, Mass.

The police say Professor Gates was arrested and briefly charged with disorderly conduct after he ignored warnings to stop haranguing an officer who had asked him for identification inside his home.