In the previous posts we looked at argument schemes that are typically employed in factual matters: generalizations, polls, general causal reasoning, particular causal reasoning, and the argument from ignorance. In this next section we'll look at common argument schemes used in normative (i.e., having to do with values) arguments. Check. it. aus...
A slippery slope argument is one where it is proposed that an initial action will initiate a causal cascade ending with a state of affairs that is universally desirable or undesirable. The implication is that we should (or should not) do the initial action/policy because the cascade of events will necessarily occur.
A contemporary example of a negative version of the slippery slope argument comes from arguments against gay marriage equality. Some opponents argue that if same sex couples are allowed to marry, then there will eventually be no good reasons against people marrying animals and so society will have to permit this too.
(Yes, people actually make this argument...can I marry my horse? )
Here's a good clip were the slippery slope argument is mentioned explicitly: Video of O'Reilly factor slippery slope argument. Start at 2:00
A positive version of the slippery slope argument might be something like a Libertarian argument (over-simplified version): We should treat the principle of self-ownership as the primary governing principle, if we do, then you will remove taxation and government, then the market will cease to be distorted and people will act in their own self-interest, and people acting in their own self-interest will pull themselves up by their own bootstraps, a society of self-pulled up people will have relatively few social problems thus eliminating many of the existing ones. (Note: We could do an over-simplified version of just about any political philosophy and show it to be weak)
So, why are these arguments not very strong? To figure it out, lets look at the underlying structure of a slippery slope argument. Recall that a slippery slope argument is one where it is supposed that one initial event or policy will set off an necessary unbroken sequence of causal events.
If we formalize it, it will look like this:
P1: If A then B.
P2: If B, then C.
P3: If C then D.
P4: If D, then E.
P5: If E, then F.
P6: (So, if A then F)
C F is a good thing, therefore we should do A. (Positive conclusion).
C* F is a bad thing, therefore we should't do A. (Negative conclusion).
(We can also condense P1-P5 as a single compound premise: If A then B, if B then C, If C then D,...)
If we think back to the lecture/post on principles of general causal reasoning we will recall that it takes quite a bit of evidence to say that even a single causal argument is good (e.g. If A then B). As you might imagine, the longer the causal chain gets, the more difficult it will become to ascertain that the links along the way are necessarily true and not open to other possible outcomes.
Returning to our examples, in the first case, one of the causal elements has to do with the equivalence between reasons against gay marriage and reasons against animal marriage. It doesn't take much imagination to come up arguments for why the two types of prohibitive reasons aren't the same (capacity for mutual informed consent for starters...). Showing that there is a relevant distinction between the types of reasons causes a break in the causal chain, thereby rendering the argument weak.
In our-simplified version of the Libertarian argument relies on a long causal chain that begins with the primacy of the self ownership principle and reduced taxation and government and ends with a decrease in prevailing social problems. Along the way there are may suspect causal claims that individually might not stand too much scrutiny--especially since many of the claims are hotly debated by experts in the respective fields. Since a chain is only as strong as its weakest link, this will have a detrimental effect on the overall strength (logical force) of the conclusion.
Upshot: So, what's the overall status of slippery slope arguments? Just like many argument schemes they can be both strong or weak depending on their constituent parts. In the case of slippery slope arguments, a strong one will have highly plausible causal claims all linked together, culminating in a glorious well-supported conclusion about what we should or should not do. Conversely, a weak slippery slope argument will have one or more weak causal claims in its implied premises.
Arguments From Analogy
An argument from analogy is when we draw a conclusion based on comparing one thing to another. Arguments from analogy are often used (but not always) to argue about a complex or poorly understood subject by comparing it to one that is less complex or better understood by the audience. They are also often used to argue for a conclusion in a controversial case based on what is accepted in an uncontroversial case by claiming the characteristics of the cases are relevantly the same.
One of the most popular analogies is between minds/brains and computers. (As an aside, it is only an argument from analogy if a conclusion is drawn. Sometime analogies are used merely as explanatory aids, not as vehicles for an argument.)
Perhaps the most famous argument from analogy is the the Argument from Design for the existence of God/gods. There are many versions of this argument, but to give us a template, here's one:
P1. The mechanics and inner workings of a watch are so mechanically complicated that they must have been designed by an intelligent being.
C1. As with a watch, life (or a particular organ or organism) is complex and has a purpose therefore, this necessitates that they had an intelligent designer.
Also, we might consider what the implications of accepting HP are. If it is indeed true (contrary to what our counter-examples show) that anything that is complex and that has a purpose also has an intelligent creator, then we must also apply this principle to whatever intelligently created life, and then again to whatever created that life, and again to whatever created that life...(you get the point!)
At this point you'll have to make some sort of special pleading argument to escape the infinite regress...
However, before we get too side-tracked, in a nutshell, the main counter to an argument from analogy hinges on showing that having one set of properties (p, q, r, z) doesn't mean that every object with properties p, q, and r will also necessarily have property z.