Again you have heard that it was said to those of old, "you shall not swear falsely, but shall perform to the Lord what you have sworn". But I say to you, do not take an oath at all, either by heaven, for it is the throne of God, or by the earth, for it is his footstool, or by Jerusalem, for it is the city of the great King. And do not take an oath by your head, for you cannot make one hair white or black. Let what you say be simply Yes or No; anything more than this comes from evil.
We are happy to conceal our attitudes and intentions with words. The Pharisees had creative ways of "swearing" that were the equivalent of saying things with their fingers crossed. Today, we want to excuse ourselves on technicalities. We withhold the truth, or are selective when we re-tell information. We spin things, presenting the facts in a "creative" way to make ourselves look good, or resorting to a non-denial denial when we are caught out.
Jesus sets the bar a lot higher than the technicalities of the words we say. What is the condition of the heart behind the words? The Pharisees righteousness went as far as saying the right thing, or not saying the wrong thing, and being satisfied with that. The greater righteousness that Jesus expects looks at the attitudes and motivations behind what we say and again we find that we are all guilty, desperately requiring the great mercy of a great Saviour.