STARING AT THE BACK OF A CAR DURING A RED light can be a mundane experience-or it can be an opportunity to think through the logic of a worldview. I recently noticed a bumper sticker that read: "AGAINST ABORTION? DON'T HAVE ONE!"

We can smirk at the logic or think through how we'd respond. This argument works when dealing with preferences. "Against broccoli? Don't eat it!" It doesn't work, however, with claims of objective morality. That's why we don't see bumper stickers that read: "Against genocide? Don't commit it!" Or: "Against rape? Don't do it!" Those slogans confuse categories. Pro-lifers are not simply expressing preferences; they are making arguments that abortion is an objective moral wrong.

The same car had another sticker slapped onto its bumper: "IF YOU CAN'T TRUST ME WITH A CHOICE, HOW CAN YOU TRUST ME WITH A CHILD?"

The virtue or vice of a "choice" is dependent upon its object and outcome. Choosing to punch a pillow is rather innocent; choosing to punch a woman is contemptible. Choosing to elect a president is good; choosing to kill him is evil. The logic of this bumper sticker makes perfect sense if you assume that the "choice" has two legitimate, positive outcomes. When that is the case, we rightly implore: "Trust me." But we don't say that when one of the choices entails something evil. Which is why we wouldn't think it profound for a young man to say: "If you can't trust me with the choice of beating my fiancé, how can you trust me with marrying her?"

Christians have our own fair share of bumper stickers with faulty logic. As Christians, we are called to mature reasoning, not childish thinking (1 Corinthians 14:20; 13:11). On the issue of abortion, the moral, legal, and scientific facts are on our side. We must master them and consider how and where and when to share them.

It has now been 31 years since the Supreme Court issued its landmark Roe vs. Wade decision. Over half of Americans think that some abortions should be legal. But virtually no one thinks that unwanted newborns may be murdered. So here's the crucial question that we should ask: Why should newborns have a right that pre-borns in the womb do not? Why should we protect one and not the other?

Scott Klusendorf of Stand to Reason, citing the work of Stephen Schwarz, has pointed out that there are only four differences between a pre-born and a newborn. They can be remembered through the acronym SLED and turned into insightful questions:

Size: Does how big you are determine who you are? Is Arnold Schwarzenegger more of a person than Gary Coleman?

Level of development: Does how developed you are determine who you are? Is the burly football player more of a person than a prepubescent boy?

Environment: Does where you are determine who you are? Does sitting inside a house make you more or less a person than one sitting outside a house?

Degree of dependency: Does dependence upon another determine who you are? Is a diabetic on kidney dialysis less of a person than those who do not need such support?

None of these differences determines whether or not you are a person. As Princeton philosopher Peter Singer writes, "The liberal search for a morally crucial dividing line between the newborn baby and the fetus has failed to yield any event or stage of development that can bear the weight of separating those with a right to life from those who lack such a right." One must either go with Mr. Singer's chilling conclusion-that "human babies ... are not persons" and that infanticide can be justified!-or reason that since there is no morally relevant difference between a pre-born and a newborn, and since a newborn has a right to life, then a pre-born has a right to life.

This is the key to answering questions like: What about rape? What about incest? What if the baby is deformed? What if she is unwanted? The best way to respond is to ask these questions about a newborn. Would any of these situations gives us a reason to kill the newborn? Of course not. But then we have returned to the original question: What is the morally relevant difference between a pre-born and a newborn?

Arguments like this cannot be easily expressed on billboards or bumper stickers, but they are key tools if we are to cultivate, by grace, what President Bush has called "a culture of life in America." Perhaps the next time you have an opportunity, you'll pray for grace, take a risk, and-with humility and hope-start a conversation.