I have been in a discussion on another forum about In Vitro Fetilization (IVF) and other artificial methods of conception or reproduction (like surrogacy), and something occurred to me; as I was articulating the teaching of the Church and the reasoning behind it, I became aware that some people would probably criticize my thoughts as being a "slippery slope" argument.
What I maintained was that once we leave off natural reproduction (through good old coitus) in favor of unnatural reproduction methods (like IVF or surrogacy), we leave ourselves vulnerable and unable to offer any kind of reasoned and logical objection to any manner of technological jiggery-pokery, like human cloning or gene manipulation or even one day growing babies in neat rows of tanks to spare mothers the trouble and risk of pregnancy.
And I believe that is a valid point. A "slippery slope" argument may be used badly or used well, and I believe that, in this case, the slope is really very, very slippery.
Or, more correctly, I don't believe it is a slippery slope at all, but something more like a blind leap off a cliff. Once natural conception is abandoned, anything must be allowed, and the only objection we will be able to muster will be "Yuck!... no one wants babies grown in tanks! That's just weird!".
And it is weird, and it seems weird... now. But if we have no solid, immovable moral principle on which to oppose it, it will happen eventually. If enough people want it (it doesn't require anything like a majority), someone will devise a way to do it. Most people, at this point in history, find the idea of Fetus Factory Farming to be repugnant, but give it time. If the only thing that keeps us from factory farming infants is The "Yuck!" Factor, that obstacle will be removed eventually, just as it has been removed for homosexuality or abortion.
Abandon natural conception as the obvious, bright moral line we must not cross, and there simply are no other lines that are not arbitrary and based on human will and even whimsy. Passing "beyond" natural conception is not a slippery slope, it is a headlong plunge off a cliff, with no branches to grab on the way down, like in the movies. Accept the God-given limitations of nature, or accept anything and everything.
The same is true, of course, of defining when a new human life begins. From a cold and dispassionate scientific viewpoint, a new human life must be said to begin at conception, when a new, discreet, living human organism comes into being. It has a new genetic code, and it is most definitely not part of the mother's body (as evidenced by the fact that it could be removed from the mother's uterus any time before implantation and be placed in some other woman's body and grow just as well).
One minute there is an egg and a sperm, and the next minute there is a new, discreet human organism... a human being by definition.
Pass "beyond" this very natural and scientific definition to more nuanced and poetic definitions, and you find again that there is no other definition of "human being" that is not arbitrary and whimsical. It's when the fetus reaches a certain size, or a certain stage of brain development, or when the blood begins to circulate, or when the fetus can feel "pain"... these things are all completely arbitrary. The best that could be hoped for would be that we would all arbitrarily agree to the same arbitrary standard... a political reality, but not a logical or scientific one.
If followed to their logical conclusions, such definitions of "full" humanity would lead us to accept that small people are less human than large ones, or that the mentally handicapped are less human than smarter people, or that we are not fully human when we are under anaesthesia, or when we are drunk. People with poor circulation would be that much less human.
There is no slippery slope, here, there is the dry, scientific reality of conception... and there is everything else on the way to going "splat". There are dozens and dozens of milestones in the development of a fetus, any one of which could be chosen arbitrarily as the definition of the beginning of a new "fully" human life, but they are all nonsense, chosen for convenience rather than from any logical or scientific process. What in the world does "fully human" mean? It could be argued until the sun goes cold, with no right or wrong answer.
So, the next time someone calls this sort of reasoning a "slippery slope", I will remember to point out that there is no slope at all... only a sheer drop. It would be little comfort if I and a friend both fell from a cliff and he were to turn to me and say, "Oh, don't worry about hitting the ground yet... this is a very high cliff and we won't reach the bottom for several seconds, now.".