Having exhausted a couple of the more obvious examples of arguments that aren't arguments at all (see posts 1, 2), I thought I'd try my hand at a trickier argument that does the rounds from time to time: the infamous slippery slope.
Is it really like the detergent-doused sheet of plastic that my brother-in-law unfurled down the hill last summer for the kids? Once you step on at the top, is it inevitable that you'll get grass burn when you slide off at the bottom?
There are three problems with the slippery slope. Firstly, it ignores reality. For better or worse, most people are hopelessly inconsistent when it comes to constructing their worldview. This means that, almost magically, in spite of the sliminess of the slope, many people stop somewhere before the bottom.
Let's take ethicist Peter Singer as the example. According to his ethic, someone is a person only inasmuch as they possess the ability to make decisions and to suffer. On this account, people with certain disabilities and people with Alzheimer's (and even newborn babies) aren't so human that killing them would constitute murder. In fact, in some cases, the greater good of society would be advanced by quietly shuffling them off this mortal coil. Of course, when his own mother suffered from dementia, Singer paid for her to be nursed until she died naturally. For all the clarity of his arguments, he ends up in fundamental contradiction.
What does all of this mean for the slippery slope? Well, Singer has gone about as far as anyone has gone in pointing out the logical consequences of a humanist ethic. But as he has done so, all sorts of people who have agreed with the foundations of his argument have balked at the conclusions. There is indeed a slippery slope here: if you start with Singer's premises, you should logically reach his conclusions. But around the world, you'll meet millions of people who start where he starts, but who stop arbitrarily at all sorts of points down the slope. The slope looked slippery, but for whatever reason (revulsion, compassion or just plain lack of courage), people stop short of the bottom-bruising end of the ride.
This reveals the second problem: if you point out the slippery slope to Fred, and then Fred steps on the slope anyway, but then somehow manages to stop himself before the impending grass burn, then he'll most likely turn to you and say, “See, you were wrong”. Fred will use the fact that the slippery slope didn't end where you said it would to justify the rightness of his position—allowing him, at the same time, to sidestep his own inconsistency.
The third problem with the slippery slope is that it's always possible to put your hand on Fred's shoulder and point to that little tussock higher on the hill and the white rabbit frolicking lower down the hill, and suggest that he must be standing on a slope. Anyone with some imagination can construct a virtual slippery slope—even if you're standing on the plain of truth. For example, we might both claim to believe in God's sovereignty in predestining people to salvation in Christ. But you sense a disturbance in my doctrine. How can I say that the gospel must be preached by living, breathing human beings without introducing human effort into the work of God and therefore launching myself onto the slippery slope that ends in Arminianism, or worse (if there is anything worse for a Calvinist)? It's even better than a mountain from a molehill; it's a mountain from a pancake (which is as flat a thing as I can think of at the moment).
But here's the final problem: sometimes the slippery slope is actually true. Sometimes decisions we make entail certain other commitments. Nobody knew when Roe and Wade were duking it out in the '70s that they were talking about the legitimacy of embryonic stem cell research, but they were. The decisions made there created a backdrop for the way that we think about human embryos, leading naturally to the conclusion that we could experiment on them.
My advice: beware the slippery slope. It's a slimy argument that's only sometimes true. So next time it's employed against you, stop and study the slope. If, on closer examination, you find a one in two gradient constructed of grease-infused Teflon, run away very quickly. But if it's garden variety concrete and you've got your rubber soled boots on, step firmly, proudly and unhurriedly onto the middle of the slope. You won't be kept standing by the magic of inconsistency, but by the grip of the truth.