I don't think anything in this life is so certain that we can know without a doubt that every promise we make we're able to keep. I definitely don't want to be the first one to cast a stone at anyone for this one. Say I promise to be at a certain place at a certain time but then someone rams my car and I don't make it. Did I really lie? NO. I had every intention and commited to keeping that promise but sometimes things outside ourselves can keep us from keeping a promise. By that thought, I'd never make a promise because nothing is 100% certain.