A Look at Slippery-Slope Arguments (Column 429)

With God’s help

Disclaimer: This post was translated from Hebrew using AI (ChatGPT 5 Thinking), so there may be inaccuracies or nuances lost. If something seems unclear, please refer to the Hebrew original or contact us for clarification.

More than once on the site, considerations of the slippery slope have come up (see for example here). This is a type of argument that people are especially fond of, and they tend to raise it in almost every discussion of halakhic policy and halakhah—though not only there. For example, a few weeks ago I was asked about a proposal to give treatment priority to those vaccinated against COVID-19. The questioner claimed that the next step would be to incarcerate the unvaccinated in quarantine camps and to deny them medical care altogether. Immediately afterward another participant asked why not also restrict treatment for smokers and for those who eat unhealthy foods and become overweight, since all of these are “to blame” for their condition (here it seems to be merely an analogy rather than a slope, and it deserves discussion). At the end of that thread the claim was explicitly raised that this is a slippery slope, and I said there that I don’t like such arguments and that I intend to devote a column to explaining why. Here it is.

Definition

An argument that aims to justify some claim is supposed to show why it is true. In a normative context, an argument for some norm is supposed to indicate that it is positive/appropriate. For example, in an ethical context one can argue that murder is unworthy because it causes the loss of life, and via the categorical imperative one can claim it is immoral because universalizing it would lead to the loss of humanity. Regarding giving charity, one can argue that this is a positive step because it improves the poor person’s situation. In the halakhic context as well, the common arguments are ones that justify a given norm. For example: women should be disqualified from testimony because Scripture states such-and-such; it is forbidden to eat the meat of such-and-such an animal because it is not on the list of kosher species; and so forth. These are source-based rationales. Rationales of “reason” can rely on the claim that this is God’s will and that we are obligated to fulfill His will, or on pointing to a problematic aspect of the specific action in question.

But there are arguments that propose a justification for some claim not on its own merits, but via its consequences. For example: it is forbidden to allow a woman to deliver a divrei Torah or to say Kaddish in the synagogue, because if we do this, in the future some will permit her to testify and to serve as a judge (Heaven forfend). What distinguishes this argument from the previous examples? It actually assumes that there is nothing wrong, in themselves, with a woman reciting Kaddish or delivering a divrei Torah. The claim is that permitting these otherwise permitted acts might lead in the future to problematic consequences. This contrasts with someone who would argue that a woman should not deliver a divrei Torah or say Kaddish because it is immodest or because “a woman’s voice is ervah”—arguments that address the matter itself. Without entering into the cogency of those latter arguments, they point to a problem in the action under discussion itself, not merely to problematic consequences of it.

Is there such a thing as a positive slope?

The term “slippery slope” is generally used as part of arguments opposing some action or measure, by pointing to their problematic consequences. But there are also “slippery-slope” arguments in a positive direction. For example: if we teach only religious studies in school, then in the future the students will attach greater importance to Torah and will continue to learn. Again, there is an assumption here that in principle it would be appropriate to teach secular studies as well, but because of future consequences we recommend a different present-day step. This is, of course, not a slippery slope in the literal sense, since there is no sliding downward but rather an ascent (in the rungs of Torah and fear of Heaven—an improvement). Still, it is an argument based on future consequences.

One can hairsplit and formulate the positive argument in negative terms: it is not worthwhile to teach secular studies because in the future the attitude toward religious studies will become disparaging or insufficiently respectful. Seemingly this is equivalent to the previous argument, but here the slope indeed descends. The concern is about deterioration rather than hope for improvement. I have already noted more than once the difference between a positive commandment and a prohibition in halakhah (see in column 202 and in columns 415 and on, and at length in the book Yishlach Shoreshav, in the essay on the sixth root), and there I showed that these formulations are not necessarily equivalent. For our purposes here, any argument that is based on future consequences will be considered a slippery-slope argument.

Is there really a difference between substantive reasons and slippery slopes? Certainty and immediacy

Another important note. There are many ordinary arguments that, on their face, look like slippery-slope arguments. For example: it is forbidden to steal because it diminishes another’s property. It is forbidden to strike because it causes pain to the victim, and so forth. These are not slippery-slope arguments, even though they have a consequentialist flavor. The reason is that the present act directly yields the problematic consequence. This is not a future concern (that may or may not materialize), but a direct and necessary result of the act (akin to the halakhic definition of a pesik reisha).

Again one can hairsplit and say: it is forbidden to shoot at another person because you might hit him and kill him—this is a slippery-slope argument. There is a chance you won’t hit him, and so the outcome is not entirely certain. Beyond the question of the probability level—which is itself relevant to the discussion—there is here the matter of directness. The killing is a direct result of the shooting and is expected to occur immediately afterward. Such a case is not considered a slippery-slope argument.

To summarize: slippery-slope arguments are characterized by the fact that the future consequence to which they point is neither certain nor immediate (somewhat similar to criteria raised by the commentators for distinguishing between grama and garmei). The relationship between certainty and immediacy requires further clarification; this is not the place for it.

A terminological clarification

Let me add another terminological clarification. The distinction between arguments that address the matter itself and slippery-slope arguments can create the impression that slippery-slope arguments are not substantive. Not so. They are perfectly substantive; I distinguish them from arguments on the matter itself only because of their nature. They are called thus because they do not deal with the matter itself but with its consequences.

Distinguishing and mixing the types of arguments: spurious correlations

There are topics for which slippery-slope arguments are used almost all the time (such as the status of women, openness to modernity, etc.). Sometimes one side in a debate raises only slippery-slope arguments, and sometimes they are raised in addition to arguments on the matter itself. When only slippery-slope arguments are raised, there are two main reasons: sometimes it is because there truly are no other arguments to support the claim; but sometimes it is meant to amplify the impression. You are forbidding an act that, in itself, is minor and innocent, or at most mildly negative; therefore you must show that in the future it will bring us to nuclear apocalypse. As is attributed to R. Ḥayyim: if I am lenient on a rabbinic prohibition, the baker’s apprentice in Paris will worship idols.

In any case, when only a slippery-slope argument is raised, the logical conclusion is that the speaker has no arguments against the matter itself (not in a pejorative sense, as above; my point is that the step, in itself, is not problematic or is even desirable). That is, in his view the step itself is appropriate and unobjectionable. Therefore, in cases where the step in question is itself neutral, it is natural to turn to questions of consequences and to raise slippery-slope arguments. But when there is a dispute over the step itself (substantive claims) and the opponents also raise slippery-slope arguments, this may reflect a tendentious and less than forthright presentation of their position.

And what about a situation in which both kinds of rationales arise? Someone claims that the step in question is forbidden and that it may also lead to problematic consequences. Seemingly this is entirely legitimate, at least so long as one is careful to distinguish between the kinds of arguments. When discussing any topic, it is important first to establish our position regarding the thing itself, and only afterward to discuss the consequences. In many cases this is not done, and it distorts and obscures the discussion.

Such mixtures are sometimes accompanied by biases. For example, there are situations in which slippery-slope arguments lead a person to oppose the step in question, and now he is also careful to “prove,” with (rather dubious) signs and wonders, that it is also forbidden in itself. More rarely, when a person concludes that the step is inherently wrong, he will also search under every stone for problematic consequences that will magnify the weight of the opposition, as I noted above (nuclear apocalypse).

I must say that I have hardly encountered situations in which someone says that the step itself is blessed—or at least neutral—but there are slippery-slope considerations that require forbidding it; or, conversely, that the step itself is problematic but has no problematic consequences. Usually all the arguments from each side point in the same direction: it is both forbidden and harmful, or it is neither forbidden nor harmful. This alignment (correlation) gives off the scent of a tendentious presentation. When all types of arguments point in the same direction, it doesn’t look “accidental.”[1] True, it is possible that this is indeed the speaker’s substantive view, but in my experience in many cases it is tendentiousness. On this point I refer you to columns that dealt with spurious correlations (see for example columns 1, 41, 151, 256, and many more).

The fundamental problem with slippery-slope arguments

Having defined the nature of these arguments, we can understand why they have an inherent problem. I will preempt here the common counterarguments to slippery slopes, which rely on the claim that the consequences will not actually arise even if we now permit the action in question. The fear of future consequences is unreasonable; it is unjustified hysteria. In many cases that is indeed the kind of response raised against slippery-slope arguments (see, for example, on Wikipedia). But for me that is not our discussion, for if this counterargument is correct, then there is no slippery-slope argument here at all—just demagoguery. Our concern here is with genuine slippery-slope arguments, namely, where the results indeed might arrive if the permission is granted.

To understand the built-in problem with genuine slippery-slope arguments, let me recall that such an argument tells us that there is a step which, in itself, is appropriate, neutral, or even positive, and nonetheless we should forbid it because of its future consequences. Let me also recall that these consequences are uncertain and not immediate (otherwise it is a substantive rationale and not a slippery-slope argument). If so, a slippery-slope argument seeks to forbid now what is permitted—or even what is desirable—because of doubt that the permission might lead in the future to problems. This means that we are mortgaging the certain present in favor of a doubtful future. Even if we are merely forbidding what is permitted (and not what is desirable), it is still a problematic argument—and all the more so if we are now forbidding something that is worthy and positive, not merely permitted. The starting point should be that one does not forbid what is permitted, for doubt does not override certainty. A very good reason is required to make us do so.

For example, forbidding women to say Kaddish or deliver a divrei Torah in the synagogue is, by all accounts, a problematic policy in itself—both because it excludes women from activities in which there is no problem for them to participate, and because these activities are important for their spiritual activity and growth. That is, these are acts that, in themselves, are desirable and worthy. Therefore, one who argues that they should be forbidden because of a slippery slope bears the weaker hand. In principle it is not right to relinquish the certain present because of a doubtful future, for doubt does not override certainty.

So what should be done with these arguments? The slippery slope of using slippery-slope arguments

Granted, there are cases in which it is nonetheless justified. For example, if the consequences are likely with high probability (the slope’s slipperiness is great), especially if they are of great negative significance (the slope is steep), and, of course, all this must be weighed against the magnitude of the present advantages of the step in question (how high is the first hill from which one would slide). In any case, my claim is that in principle the burden of proof in such cases lies with the one who seeks to forbid. So long as heavy-weight reasons have not been brought, such actions or steps should not be forbidden.

David Enoch devoted an article to slippery-slope arguments (this also came up here on the site). Already in the title he writes that there is a slippery slope in our use of slippery-slope arguments. It is precisely this formulation of his (which I have mentioned more than once) that I do not accept. If a slippery-slope argument is not admissible (because of the relative weights described above), then do not accept it. Rejecting it with the claim that there are slippery slopes is to reject a good slippery-slope argument because of concern about future bad ones. Once again, this mortgages the certain present in favor of a doubtful future.

Tolginos, in that thread, proposed the following compromise: Enoch prefers a policy that categorically rejects slippery slopes to a policy that always requires them. But of course both approaches are poor, and there is no reason to adopt either. As I explained, we should give these arguments a place—but with caution and suspicion. I have mentioned here in the past a saying attributed to Rav Kook (I was previously corrected that it is attributed to the Rebbe of Vizhnitz, but I have now been told second-hand that it is indeed Rav Kook): It is better to err on the side of gratuitous love than on the side of gratuitous hatred. To which I add: best of all is not to err in either. When there are two bad options, that does not mean we must choose the less bad. Better to look for an option that is better than both.

Between slippery slope and vagueness

In many cases I have seen people use the term “slippery slope” for situations of vagueness. For example, some argue that abortion should be forbidden because a fetus is in an intermediate state between a human being and an animal (it lacks consciousness, thought, will, etc.). This is a conceptual error, since such an argument aims to point out that the concept of life is not binary—that is, there is a continuum of degrees of “aliveness” between a plant or an animal and a living human being—and therefore we should not permit the killing of a being in such an intermediate state, because we have no way of knowing where along that continuum the states begin that are already worthy of the designation “human life.” Note that this argument does not rely on future consequences, and therefore it is not correct to call it a slippery-slope argument. It points out that we have no way of knowing when the being is already considered a person whom it is forbidden to kill, and therefore out of doubt we should be stringent along the entire slope (even if it is not “slippery”). This forbids the present state (albeit perhaps more leniently), rather than forbidding something permitted for fear of future problems.

My assumption here is that in such cases one should use fuzzy logic—namely, there is a continuum of degrees of “aliveness,” from which follows a continuum of degrees of prohibition (degrees of killing). One can, of course, think of cases in which the boundary exists but is unknown (an epistemic rather than ontic doubt; see the series of columns 322 and on), and there it is more similar to a slippery-slope argument: our ignorance will cause us to slide down the slope and reach an impermissible state.

What causes confusion regarding fuzzy logic is probably that even in such cases one can see, in the background, a fear of a slippery slope. If there is a vague state with no sharp line distinguishing a person from a plant or an animal, then the concern that permitting the killing of a fetus will lead to the killing of a person is greater. For example, we will permit killing a one-month-old fetus, and others will come to permit killing a five-month-old fetus; and perhaps there will even be those who permit killing an infant (who likely also lacks genuine will and awareness, etc.). A continuum of vague states indeed constitutes a slope that is easy to slide down. But presenting the problem as a slippery slope misses an important point: even regarding the one-month-old fetus itself there is no clear permission to kill it. Therefore first of all it is important to point to the intrinsic prohibition, and only afterward to add the slippery-slope consideration. As I noted above, raising a slippery-slope argument carries the subtext that there is no problem in killing this fetus in itself.

There are sometimes situations in which there is vagueness in the definition (an ontic doubt—the first type defined two paragraphs above), yet there is still a line—usually not sharp (the sorites paradox)—that distinguishes between problematic and non-problematic states. For example, a person may argue against licensing a mosque in a city for fear that we will be flooded with requests to establish additional mosques, changing the city’s character. Let us assume for the sake of discussion that the existence of a single mosque in the city is not problematic at all, but a hundred mosques could already be a problem. In the discussion about the first mosque there is only a slippery-slope claim, since it is not problematic in itself. On these assumptions, this discussion contains no argument on the matter itself, only a slippery-slope claim.[2]

In such cases the vagueness itself is a significant factor in the likelihood of sliding down the slope. If we begin by permitting the first mosque, this can indeed lead to permitting more. Partly because if there is no problem with one mosque, then logically there should be no problem with two or with a hundred (the sorites paradox). Wikipedia lists three mechanisms that can lead to sliding down the slope: first, the principle of equality; second, the principle of momentum (the permission will motivate further requests); and third, the principle of kinetic friction. In mechanics it is known that the friction the ground exerts on a moving body is smaller than that exerted on a stationary body. Similarly, once we have permitted one mosque (we are “in motion”), it will be easier to continue permitting those that follow.

A value-consciousness slippery slope

Sometimes one hears value- or consciousness-based slippery-slope arguments. In the example mentioned above: if we restrict treatment for the unvaccinated, we will come to restrict it for the overweight; or: if we permit homosexual relations, we will come to permit incest within the family. These cases are somewhat different from the previous examples, since what is at issue is the deterioration of norms and not only of behavior. In addition, we are speaking about the deterioration of a society, not of a single individual.

To sharpen this, let me take the second example. Suppose that if we permit homosexual relations, then later we will also permit incest. Here it is not that we will fail in a forbidden act, but that our norms will change. But if, at some future stage, we indeed think that such an act is permitted—then what is the problem? The problem is that now we think it is forbidden. In other words, we are looking at the future through a present-day lens. Yet this is a very problematic argument. If then we think that such an act is permitted, why impose our present point of view on the future? This is not a concern we should act upon. If there is a concern that we will fail in a forbidden act—that is, a psychological-behavioral slippery slope—then that is a relevant argument. But a value- or norm-based slippery slope, in my opinion, is a null argument.

For this reason I am in principle opposed to a constitution in this draconian sense. Think of a constitution that contains laws that require a supermajority to change—say, 70% of the Knesset. Our present-day determination binds the hands of a future Knesset. Suppose that in five hundred years there is a 60% majority in the Knesset to change the law. They will be unable to do so because today there was a (perhaps accidental—but even if not) majority that set a 70% requirement. Why should today’s legislators dictate the conduct of the state in a hundred or a thousand years when then there is a solid majority against it?

In halakhah there is the notion of nedarei ziruzin—oaths of encouragement. A person swears to perform a mitzvah; strictly speaking, such an oath does not take effect. We are already sworn from Sinai, and an oath does not take effect on an existing oath. Why would anyone swear such an oath at all? If he is going to perform the act despite its prohibition, then he will also violate the oath—so what does swearing help? The matter is apparently psychological (we have strong aversion to violating an oath). But it is also possible that the person fears a state in which he will think in the future that the act is not prohibited (he will change his halakhic view), and that is what he seeks to prevent by his present oath. Yet in that case I do not understand why to take a present step that will bind his hands and not allow him to act as he will then understand? If in the future he thinks it is permitted, then he should act according to what he thinks at that time, not according to what he thought ten years earlier.

This reminds me of the well-known story of R. Naḥman of Breslov about the king and his friend who knew that the whole world was about to go mad (eat maddening grain) and decided to write on their foreheads “I am mad,” so that after they went mad they would at least be aware of it. If you are about to go mad, this has a certain logic, because clearly your present view has an inherent advantage over what you will think in the future. But if you fear a future change of opinion, then there is no logic to writing your present view on your forehead. It is no better than what you will think then. On the contrary, we have a Talmudic dictum that the halakhah follows the later authority (halakhah kebatra’i), because he is familiar with the arguments of his predecessors.

In short, a slippery slope should relate to the psychological-factual plane and not to the value-normative plane.

An example of a slippery-slope argument

I will now bring a passage whose analysis allows us to use the entire arsenal presented so far. A man named Prof. Davidson raised the following argument in a Knesset committee meeting (from Wikipedia):

I want to bring a story, though I ask your forgiveness for the comparison: in the early 1930s there was severe distress in Germany, which also affected the medical field. It was impossible to provide medical treatment to all the sick. The German Medical Association then advised physicians: do not treat every patient, but only those who can be cured within a short time. A year or two later, another decision was taken by the German Medical Association: do not treat patients whose opinions stand in sharp opposition to the government’s view. Some time later another decision was taken: not only will we not provide treatment, we will help the government eliminate those whose opinions do not accord with the government’s. In the Nuremberg Trials there participated an observing physician named Leo Alexander, who wrote about the steep slope you descend the moment you begin making decisions that render one person’s life worth less than another’s.

The first proposal was very reasonable. It seems the appropriate and sensible policy for that situation. And in any case, if someone thinks it is not reasonable, then he need not resort to slippery-slope arguments. But Davidson presents here a slippery slope (which in fact occurred), whose next step was not to treat patients with problematic views (in the government’s eyes). This is, of course, not a necessary result of the first decision, and the justification for it is unconnected to the reasonable justification for the first proposal. One might think that the first decision lowered the sensitivity threshold regarding the duty to treat patients, and then perhaps there is a fear of a slippery slope. I am inclined to think that the causal relationship was the reverse: because the sensitivity was low, they slid down the slope. Moreover, if the Nazis concluded that there is indeed no justification to treat those with problematic views, I do not understand what slippery slope he found here. That was their view at that stage, and they acted accordingly. This is a change of norms rather than a mere behavioral change.

At the next stage down our slope, these fellows decided to eliminate those with opposing views. Yet this actually proves the correctness of my analysis (that there is no slippery slope here). The Nazis changed their values and, at some point, denied the value of the lives of those with opposing stances. This has nothing to do with slippery slopes, since it is a change of values, not merely of behavior. This argument is such typical demagoguery that I could not resist presenting it here.

Note that moral pathos leads us to identify with the words, but we do not notice that this is demagoguery. Davidson (following the physician Leo Alexander, whom he mentions) exploits the fact that the Nazi stance itself repels us, in order to feed us a baseless slippery-slope argument about the value of life. Does he expect that when we lack sufficient resources to treat everyone we will not set prioritization criteria? What, then, is his alternative?

This is a typical and very common demagogic method (see, for example, Justice Elon’s words quoted at the end of Rubinstein’s article here, near note 23). One presents a conclusion that elicits the listeners’ identification, even though the argument that supposedly grounds it is flawed and fails. Identification with the conclusion distracts from the flaws in the argument. I have often noted that one can have a poor argument leading to a true conclusion, as well as a false conclusion arising from a valid argument (if not all its premises are true). We must beware of such deceptions.

The slippery slope in halakhah

In halakhah as well there is, of course, room for slippery-slope considerations; but there too it is very important to distinguish them from arguments on the matter itself. A significant portion of rabbinic prohibitions (derabbanan) are based on fears of future consequences and not on substantive considerations. Fences and decrees (seyagim and gezerot) were enacted to distance a person from transgression—that is, we forbid a present-day step that is permitted, and perhaps even desirable, out of concern that permitting it will lead people to future transgressions. The term “fence” (seyag) in its literal sense is a barrier set up within the permitted domain to prevent crossing into the forbidden domain.

Take an example: riding a horse on Shabbat was forbidden lest one come to pluck a branch. Riding, in itself, is permitted, but there is concern that the rider will come to the Torah-level prohibition of reaping (ketzirah), and therefore the Sages forbade riding mi-derabbanan. Likewise, in the first mishnah of Berakhot, they forbade reciting the evening Shema after midnight, lest the permission lead a person to fall asleep and forget and not recite it at all.

The prohibitions of “appearance’s sake” (mar’it ayin) and “ways of peace” (darkei shalom) in halakhah can be interpreted as slippery-slope arguments, since they too are based on future concerns. This is at least their common interpretation, though each deserves discussion. The same goes for various “emergency measures” (hora’at sha’ah).

Elyakim Rubinstein, in his article on emergency measures and the slippery slope in halakhah, notes that the Sages hedged the mechanism of slippery slopes with the rule that one does not enact a decree on top of a decree (ein gozrin gezeirah legezeirah). There is here a reflection of the claim that there should not be a slippery slope in the use of slippery slopes—which, as we have seen, is an imprecise formulation of the concern about forbidding the permitted; that is, mortgaging the certain present for a doubtful future.

Halakhic implications of what we have seen

In the halakhic context as well there is attention to the distinction we saw above between a vague state and a slippery slope. For example, see note 1 above regarding Rav Kook’s claim about the inherent prohibition in every fence. He identifies the two types of considerations (in effect, he does not recognize the slippery-slope consideration on its own), and I already pointed out the difficulty in his words.

Beyond this, in the laws of Shabbat (and not only there) later authorities distinguish between two types of rabbinic prohibitions. There are those that are expansions of a Torah prohibition (de’oraita), and those that are merely fences around a Torah prohibition. For example, the rabbinic prohibition on selecting food from refuse (borer—Torah-level borer is only selecting refuse from food) can be understood as an expansion of the Torah prohibition of selecting. This is a lighter prohibition because it does not cross the Torah threshold, but qualitatively there is some “selecting” going on (somewhat like a “half-measure”), and therefore the Sages forbade it. See column 257 near note 6. By contrast, the example of riding a horse is the most clear-cut example of the first type: it is obvious that riding is not an expansion of the labor of reaping, since there is here no qualitative reaping, even a weak one, but rather something entirely different that might lead me to reap. That is certainly a pure fence, not an expansion of a Torah prohibition.

This appears almost explicitly in Maimonides, Hilkhot Shabbat 21:1:

It is said in the Torah (+Exodus 23+): “You shall rest”—even from matters that are not a labor; one is obligated to desist from them. And there are many matters that the Sages forbade because of shevut. Some matters are forbidden because they resemble the labors, and some matters are forbidden as a decree lest they lead to an offense punishable by stoning. And these are they.

What is the difference between these two types? The first type is a pure slippery slope: we forbid something entirely permitted only out of fear of a future problem. The second type concerns an action that carries some faint prohibition (that does not cross the Torah threshold), and the Sages forbid it on that account. In such cases, beyond the inherent problem in this action, it may also lead to a Torah-level problem, and therefore there is also a slippery slope, since, as I explained above, vagueness can itself cause sliding down this slope. Note, though, that in halakhah the prohibition on selecting food from refuse—which is rabbinic—does not have the same status as selecting refuse from food—which is from the Torah. That is, the real line is not vague, and the Sages know how to draw it. Yet there is concern that ordinary people will not know how to draw it and will fail.

The slippery slope as the seamline between Torah law and rabbinic law

I will conclude with a fundamental observation that, to my mind, many are unaware of (I noted it in a lecture here, and also in responsa here and here). There are enactments—such as the shevu’at hesset (supplementary oath), the market regulation (takkanat ha-shuk), and others—their rationale is not a phenomenon that suddenly appeared on the stage of history. They are not a response to a situation that arose, but a sevara (reason) that likely existed always. Regarding the shevu’at hesset there are even hints that it is an ancient law that appears already in the Mishnah, although in the world of learning its nickname is “an oath of the Gemara” (as opposed to Torah oaths and “Mishnah oaths,” most of which are listed in the Mishnah at the beginning of chapter 7 of tractate Shevuot). Later authorities (such as the Beit HaLevi, the Ḥatam Sofer, and others) wonder when these enactments were created and even assume that if it is an enactment, then it certainly cannot be from Sinai.

But in my opinion there is a misunderstanding here. The laws of the Torah deal solely with what is inherently forbidden or permitted—with intrinsic obligation. If there is room to forbid or to command something only because of considerations of future concerns, etc., that is the role of the Sages. The market regulation is meant to stabilize commercial relations and introduce certainty. That is an interest that is always valid, and it is unlikely to be a response to some situation that arose at a particular point. Therefore it is entirely possible that it is an ancient law—perhaps from the time of Moses (there are enactments attributed to Moses), or even earlier (the prayers are an enactment of the Patriarchs: “the Patriarchs instituted them”). And yet it is a rabbinic law, because its nature is that of a concern, not of an intrinsic rule. Inherently it is clear that if an object belongs to so-and-so, then even if I bought it from another in good faith, I must return it to the owner. The object is his. But such conduct would introduce unbearable uncertainty into commercial life (one cannot check the chain of title for every object; even for real estate, which is registered in the land registry—and previously in the rabbinical court—it is very complicated and problematic). Therefore the Sages decided to transfer ownership to the buyer, even though it was not his previously. By its very nature this is a rabbinic law, since it does not express a juridical truth but a future concern.

From this perspective, slippery-slope arguments belong by their very nature to the sphere of rabbinic law. In the Torah sphere such arguments are not relevant. Hence later authorities indeed assume that there are no Torah-level fences (see Lekah Tov by R. Yosef Engel, Principle 8, who discusses this). From this perspective, slippery-slope considerations are part of the watershed between Torah law and rabbinic law (regardless of chronology).

This again raises the question of what could be the rationales for the Torah’s commandments (which, although we do not derive them into practical law, are very likely to exist). Seemingly every rationale involves a future concern: we forbid some action because it is problematic for some reason; we command some mitzvah because it produces some benefit (perhaps spiritual). So what is the difference between Torah law and rabbinic law—aren’t both based on future concerns? Here I can employ what we saw above: if the concern is certain and immediate—i.e., it arises from the very act—then it is not a slippery slope. I gave, as an example, that murder is problematic because it brings about the loss of life. This is not a slippery-slope consideration, for the result is immediate and clear from the act itself, not a future consequence in the sense of a slippery slope. Once again we see that understanding the nature of slippery-slope arguments illuminates the difference between Torah laws and rabbinic laws.

[1] Rav Kook, in his Musar Avikha, argues that if something is forbidden as a fence, this means that doing it can lead to problematic consequences, and that itself is an indication that there is some problem with it in itself. This is, to me, a very strange claim. If that is the case, the Sages should have forbidden it on its own merits and not relied on fences. For halakhic implications—see below.

[2] See on this in columns 252–253.

[3] Of course this is not the entire line: there are substantive rabbinic laws (such as the enactments of Ḥanukkah and Purim), and according to some views (cited in Lekah Tov there) there are also a few Torah-level fences.