Suppose I have promised to keep a confidence and someone asks me a question that I cannot answer truthfully without thereby breaking the promise  Obviously, I cannot both keep and break the same promise
Ttherefore, one cannot be obliged both to answer all questions truthfully and to keep all promises
Some politicians could attain popularity with voters only by making extravagant promises; this, however, would deceive the people. So, since the only way for some politicians to be popular is to deceive, and any politician needs to be popular, it follows that some politicians must deceive.