Fools love thoughtless vows as well. Talking about what you will do is a good, low-cost way to enhance your reputation down at the church. Think of a similar tactic employed by the late Ananias and Sapphira. This section of Ecclesiastes addresses a financial pledge, and then when a temple messenger comes to get it, the story told by the one who vowed is all different now (vv. 4–7).
