He seemed to me to be criticizing people who want to make changes to the code without reasoning about the system, assuming that if they mess something up the ex

He seemed to me to be criticizing people who want to make changes to the code without reasoning about the system, assuming that if they mess something up the existing tests will catch it and tell them. I think he is criticizing a real problem, but the metaphor is all messed up and hence misleading. Writing software is not like driving a car. It is like engineering a car. Making a change on the assumption that tests will tell you if there is a problem is like deciding to move the gas tank and saying, "If that turns out to be an issue, QA will tell us when they do their collision tests." (Sort of like that. Obviously less expensive.) When you are making a change, you need to think about whether that change will require any new tests and you need to be thinking about how it integrates into the system as a whole. Maybe moving the gas tank means a new kind of collision test needs to be added to the repertoire. Because the metaphor was bad, the GP seemed to be taking it as a criticism of testing, which isn't reasonable. But I may have been mistaken about his intent. On the other hand, if the change you are making is just altering the shape of the bumper for cosmetic reasons, it seems pretty reasonable to assume that QA will tell you if it adversely impacts safety in ways that were not obvious. I agree about static types. Static types are basically a way of having the compiler automatically write and run whole classes of unit tests.