Thou shalt not bear false witness against thy neighbour.
The Ninth Commandment is popularly abbreviated as "Thou shalt not lie", but I will explain why the truth behind the command is a bit more complicated than that.
First, is "bearing false witness" the same as "lying"? The Hebrews understood the phrase to be a formulation specific to official court settings, prohibiting "swearing falsely against your neighbor in matters of law and civil proceedings". Being generally honest and truthful is good, but such a broad interpretation of the Ninth Commandment is unsustainable.
There are clearly situations in which it is acceptable -- even laudable -- to deceive and lie. (Hat-tip to SS for these.)
- Hebrew midwives lie to Egyptians to protect baby Moses
- God tells Samuel to lie to Saul about his reason for coming to Jerusalem
- God helps Elisha lie to an Aramite army
- Jael tricks enemy general Sisera into her tent, then hammers a tent peg into his head
- God ambushes Ammonite and Moabite armies
- God commands Joshua to ambush Ai
Although our European and early American forebears believed that combat by stealth was dishonest and dishonorable and that soldiers should line up and fight face-to-face across a field of battle, God apparently disagrees. Furthermore, when playing a game or participating in a purposefully adversarial system it is not only acceptable to deceive, but expected. When playing basketball, it is not a sin to deceive by "feinting" one direction and then running another.
So it makes sense to interpret the Ninth Commandment narrowly and to understand the purpose God had when he wrote it. The Catechism of the Catholic Church explains it well:
False witness and perjury. When it is made publicly, a statement contrary to the truth takes on a particular gravity. In court it becomes false witness.When it is under oath, it is perjury. Acts such as these contribute to condemnation of the innocent, exoneration of the guilty, or the increased punishment of the accused.They gravely compromise the exercise of justice and the fairness of judicial decisions.
Second, what do the above scenarios -- in which it is acceptable to lie and deceive -- have in common? I think the key is to be found in the oft-ignored, oft-omitted final phrase of the commandment: "against thy neighbor"... begging the question, "who is my neighbor?". How helpful of Jesus to answer that question for us!
The parable of the Good Samaritan is often interpreted to mean that everyone is my neighbor, but that's not what Jesus says. The Good Samaritan was "good" because he had an opportunity to be merciful, and then was merciful. The priest and the Levite were "bad" because they had the same opportunity and did not show mercy.
I don't want to read too much into any particular passage or twist myself in philosophical knots, but it appears to me that an underlying assumption of the Ninth Commandment is that you need to be truthful when there is an expectation that you will be. When you swear an oath, when you publicly assert a fact, when you make a business deal, or when you deal with an intimate acquaintance there is an expectation on the part of the listener that you are being truthful. The listener will act on and rely on what you have said, and if you are lying he will be injured.
In contrast, your wartime enemy and your basketball opponent have no expectation of honesty from you within the context of your confrontation: they are not your neighbors. Classically: the Nazis coming for the Jews in your attic are not your neighbors.
So the greatest part of discernment appears to rest on this final phrase, "against thy neighbor". A fool will be quick to define his neighborhood very narrowly, so as to permit a wide range of deception. Returning to the parable of the Good Samaritan, however, we see that Jesus intends us to define "neighbor" broadly. The parable is surprising specifically because the Samaritan was the traveler with the least expectation on him. Everyone knew that the Samaritans and Jews disliked each other. Of the three, the priest and the Levite were supposed to be the most spiritual, the ones everyone expected to help a person in need along the side of the road. But the Samaritan was the one who defined his "neighbors" broadly and who was consequently given the title "Good", a commendation which has resonated for 2,000 years.