The "slippery slope" argument format (also known as the "camel's nose in the tent," the "give an inch," the "crack in the foundation", and other names) is essentially that if you make any exceptions to a rule, or if you make rules that depend on fine distinctions, pretty soon people will be ignoring the rule or rules entirely because they won't accept the difference between the exception and everything else. "If you allow exceptions to a rule, it creates a slope away from the absoluteness of the rule, down which people will slide further and further until they will not obey the rule at all." "If you give people an inch, they will take a mile" or "If you let the camel put its nose into the tent, pretty soon the whole camel will be in your tent."  As I write this the most recent application I have seen of the argument is that "if we allow embryonic stem cell research, which sacrifices early-stage embryos, the next thing will be that infanticide and euthanasia of the terminably ill will be permitted so that we can use their body parts for research or cures.  If you don't hold all life to be sacred, then no life will be held to be sacred."

The slippery slope argument is clearly invalid if it is meant to be a point of logic, for it does not follow that "if b is an exception to A, then no part of A is true."  Specific exceptions to a rule or principle do not in any way logically imply that the rule is otherwise false or never justifiably applicable in any cases.  In fact, calling something an "exception" points out that only it is the relevant act that the rule does not cover.  If, for example, a pharmaceutical drug should be used only by people who have asthma, that does not imply people should also take it for arthritis or pregnancy.  Permitting stem cell research on embryos does not logically imply that sacrificing infants or terminally ill patients is acceptable.

It appears the argument is meant to be more an argument about people's psychology, and, spelled out, it seems to be something more like "if you make any exceptions to a rule, particularly a cherished or time-honored rule, people will think the rule arbitrary to begin with and will see no reason to follow it at all."  Hence, any exceptions undermine respect for a rule, and thus eventually lead to the rule's not being followed at all.  Or another intended argument might be "people cannot generally make fine distinctions, so if you make an exception to a (time-honored) rule, people will think you have shown the rule to be flawed and therefore unnecessary to follow." 

A slightly different, and more sophisticated version of the principle might be "if you make exceptions to a rule, people will generalize the reasons for that exception and apply them to other aspects of the rule to which those generalizations will also apply."  In the embryo issue, the argument would be that "if you allow embryonic stem cell research people will see that defenseless human life has only instrumental value --value for helping others-- so nothing will stop people from wanting to kill infants or people with terminal diseases to help others."  Or it might be phrased as "if you allow embryonic stem cell research because embryos are not viable on their own, then you will end up allowing infanticide and termination of the lives of the terminally ill because they are not viable on their own either."  I will deal with this kind of version of the slippery slope argument last.

Also there are people who break rules because they see other people get away with breaking them, and think that it must be acceptable.  This too has nothing to do with fine distinctions or exceptions in rules. 

Now, it is true that if there are no laws governing a certain kind of business practice, one company's legal transgression of a "latent" ethical practice or social more will often encourage other companies to do the same or go even "further."  But that is a different kind of case.  That is a situation where (business) people tend to believe either or both of the following statements -- statements which are both actually false:
1) whatever is legal (i.e., whatever is not illegal) is moral and right to do.
2) if there is no stated (accepted) moral prohibition against something, it must be moral.

The second proposition is false because there can be, and often is, a difference between what is moral and what is thought or said to be moral.  This is similar to the difference between what happened in the past and what gets put into history books; the two are not necessarily the same thing.  Another analogy is math: the correct answer to a math problem is not necessarily what someone says or thinks it is, and there is a correct answer whether anyone knows it or not.  Morality is what ought to be the case, whether anyone knows it or not or has said it or not. 

The first is false because laws tend to be made when legislators think there is some necessity for them.  Legislators do not try to put into law every moral or social principle or value they know.  If no one in a society has ever committed a particular act, and no one is ever even imagined to possibly commit such an act, there is not likely to be a law proscribing the act.  Even when there are laws prohibiting certain general categories of acts, such as obscenity, it often requires a court decision to declare whether a particular act falls into a given category or not. 

Even though there are a great many laws, no one can reasonably think that laws are co-extensive with morality or moral principles.  For example, there is much concern with how the rapid pace of technology outgrows settled law as totally unanticipated business practices (such as sharing music online) become possible and put into use.  Or fifty years ago, no one thought it was necessary to make laws involving petri dish or test tube embryos or their parentage; and before cloning became imagined and possible, no one thought to create laws prohibiting it.  This applies not only to technology but to services as well. Even if there were laws prohibiting the selling of babies, would that mean it is not allowed for attorneys or adoption agencies to charge huge sums to process the work involved in adopting children, or is that just a mere business opportunity?  Until the costs became substantial for adoption, legislatures and courts were not faced with having to draw lines or get involved in this sort of issue.  As new ways are invented essentially to circumvent old laws, legislation is often passed to shore up the loopholes or prevent the circumvention.

Moreover, we also do not tend to state even all the moral principles, or particularly their implications, that we have.  When parents tell their battling children to quit hitting each other, they are not giving implicit permission for the kids then to start kicking or spitting at each other or trying to stab each other with pencils or scissors.  The general principle might be to "stop trying to hurt each other" but even that needs modification so that some necessary hurts might be permitted or necessitated -- as in putting a painful antiseptic on an open wound, or having to make a tourniquet or an incision for a snake bite, etc.  A parent might even permit certain forms of retaliation for an unjustified and hurtful attack.  We have a whole repertoire of moral principles or moral ideas by which we make decisions without spelling them out even to ourselves, unless we face some situation that requires us to try to articulate the rule by which we are operating or making a decision.  Even when we try to articulate our reasons, we often do not cite the reason we actually use.  For example people will often cite the Golden Rule for doing something helpful for someone else, but the actual reason was that they wanted to help the other person, not that they would want the other person to help them.  When you give first aid to someone, you don't ask whether you would want first aid; you simply see they are hurt and need help that you know how to give and can give.  Even if you try to reason through when to give first aid and when not to try, it will end up being far more complex than just thinking about what you would want to have happen if you were injured.

In general we often do not spell out all our moral principles but draw upon them or discover them as we need to.  The world was revolted by the Nazi genocides but until the Nazis did it, no one thinking that a civilized country would do such a thing felt the need to say "genocide is wrong" even though if anyone were asked, they would think it was.  We just do not go around, even as a society, stating all the principles or acts we can think of that would govern all possible behavior.

There are many possible things that are immoral which are not illegal simply because no one ever suspected they would be done and therefore thought it necessary to pass a law to prohibit.  And there are many immoral things which no one ever suspected need to be stated because no one ever imagined anyone's doing them.  It is simply false that everything is moral which does not have a law against it; and it is also false that anything is moral which no one, or no culture or group, ever said was not.  New laws have to be, and get, initiated and passed all the time because someone does something no one ever previously would have thought needed to be proscribed, even though if they had imagined someone actually thinking of doing the act they would have recognized immediately they thought it would be very wrong to do.

Moreover, there are even laws from time to time that people argue are immoral and need to be amended or abolished. Often the moral argument will win out, and the laws will be changed or abolished because they will have been seen to be immoral.  The law, and conventional practices, do not always reflect at any given time what considered moral analysis might show to be right and wrong.  What is merely legal is not always, nor necessarily, moral.

In short, business practices that cross ethical boundaries often do so for reasons other than that laws are passed which have exceptions or which depend on reasonable distinctions.  There are, of course, laws which make distinctions that are either unreasonable or that only a specializing attorney would likely know, and such laws may easily be transgressed by unsuspecting people who do not realize they are violating a law.  And there are laws that get transgressed by good people who think they are bad laws that do not deserve to be obeyed.  But again, these case have nothing to do with supposed slippery slope behavior or slippery slope arguments.

Slippery slope reasoning is, by itself, in fact, almost never feared.  If it were, we would have to worry about large segments of the populations always running red traffic lights, since they are just like green traffic lights except for their color, and if you can go through green lights, why not red ones.  Or we would have to worry about wholesale adultery since if being married allows people to have consensual sex, why should it matter who it is with.  And, if people really used slippery slope reasoning, they would have no regard for property rights, since if owners can do with their property as they wish, why not everyone else!  Slippery slope advocates would have to argue: Once we start on the slippery slope of allowing men to have sex with one woman (their wife), it is just one more step down the slippery slope till they will be having sex with all women or any woman.  Once we let someone do what they want with their own property, it will be just a matter of time before they start doing what they want with other people's property.  Once we let people go through a green light, it is just another step on the slippery slope till they are going through red lights en masse. 

But the fact is that normally we neither use slippery slope reasoning nor fear its being used.  Normal people and the courts are generally perfectly capable of making distinctions between one kind of case and another, especially when relevant differences are pointed out to them.  In important social issues, it is highly unlikely that one kind of case will be treated like another just because they are alike in a few ways, particularly a few trivial or superficial ways.  If they are treated alike at all, it will because they are considered to be similar in morally relevant ways or because they each have their own (different) rationales that justify the same treatment.  That does preclude mistakes in such reasoning from being made.  But there are far more likely and prevalent errors that lead to such mistakes than mere slippery slope ones.

At this point I wish to consider the more subtle version of the slippery slope argument -- that if you make exceptions to a rule, people will generalize the reasons for that exception and apply them to other aspects of the rule to which those generalizations will also apply.  I gave two examples above.

The problem here is not the practice of generalizing from the reasons for an exception; the problem is generalizing from the wrong reasons for the exception, or generalizing from reasons or principles that should not have been the rationale used or given for the exception in the first place, assuming, for argument's sake here that the exception is a justified exception.  This kind of situation arises either (1) when a faulty justification is given for the exception that is permitted and people accept that as the actual justification, or (2) when people incorrectly infer what the justification for the exception is.  In the above embryo research case, neither of the two rationales stated (and commonly inferred by opponents of stem cell research) is the actual or best rationale given for permitting embryonic stem cell research.  The actual argument probably would be something more along the lines that have to do with lack of sufficient cell differentiation for the embryo to have ever attained anything like personhood, self-awareness, or any level of consciousness -- in short, with nothing of the sort yet having been formed that makes life in any way worthwhile or something to be clung to or desired by the cells in question -- that although they have the potential to become a human being, they are not yet a human being, and that they are closer in relevant ways to the egg and the sperm cells that were just mated, rather than to the human being that they could become if implanted in a womb successfully.  And since we do not consider unfertilized eggs or sperm cells under normal conditions to be a loss if not turned into living persons, we should not consider it a loss if embryonic cells are also not permitted to become human beings or human persons, particularly since the point of the research is to help already living humans who have put in some effort and work in life be able to reap a reasonable amount of the benefits of that work.  The embryos are not being created or destroyed for no reason; there is an important benefit to be gained. That is quite different from saying that it is okay to kill already formed humans who are no longer viable or who are defenseless.

Unfortunately in our society today, few full, rational explanations, justifications, or arguments are ever spelled out in their entirety.  Politicians and legislators do not do it; at most they repeat their own side and ignore the opposition's and the opposition's objections, or treat them superficially. The news media will normally not print long, detailed, intricate or involved explanations if they were given.  And even Supreme Court decisions, which contain rationales fully available in print, give only partial arguments since dissenting opinions are published but not taken into account, thus likely leaving flaws in many of the majority opinions. Sometimes it is left to legal scholars to "flesh out" the likely fuller, or perhaps unrealized, arguments for, or "standards implied" by the majority position. A complete rational argument must not only fully explicate the reasons for a position but must also account for the problems with the opposing view, not just ignore them.

It would be better, perhaps particularly for future generations whose circumstances may be different, for there to be full explanations of exceptions and fine distinctions in making laws or policy, so that inferences would not be necessary, and so that important distinctions could be seen and not as likely discarded by error or lack of understanding.