A well-designed device should not injure a human being. If possible, it should not allow a human being to come to harm through its inaction. The later is the difference between a car and a scissors. I expect a car to have safety devices. I don't expect a scissors to stop me from running while I hold it, if I'm stupid enough to do so. Devices should only be expected to work at their general level of complexity. A well-designed device should obey the direction given to it by human beings, except where such direction would harm the human using it. I want it to be hard for me to cut myself with those scissors, but I want those scissors to cut everything else well. A well-designed device should protect its own existence, so long as such protection does not cause damage to the human being using it, nor impair its utility.