I. General information on slippery slope arguments

The slippery slope argument is a figure of reasoning by which an action that seems morally acceptable is conclusively shown to be morally impermissible in reaction to its morally unacceptable consequences. In general, two variants of slippery slope arguments can be distinguished based on two possible consequences: the causal variant and the conceptual variant.

The causal variant postulates that the introduction of a practice that is morally acceptable will, with a certain probability, also lead to the establishment of a morally unacceptable practice in the future. Therefore, contrary to first appearances, the practice that is morally acceptable had better not be introduced. A recurring weakness of causal slippery slope arguments is that in most cases it is difficult or impossible to provide serious and accurate data on probabilities for the occurrence of the undesirable consequence. Even if plausible probabilities can be identified in individual cases, a causal slippery slope argument is only valid if the negative consequence is both sufficiently weighty and if its probability of occurrence is high enough to outweigh the foreseeable benefits associated with the introduction of the morally acceptable practice.

The conceptual variant of the slippery slope argument often occurs in a certain variety, the so-called vagueness variant: If action (1) is permissible, then an action (2) that deviates minimally from it is also permissible, which in turn justifies an action (3) that goes beyond it, and so on. Thus, one finally arrives at an act (n) that is morally unacceptable. But since act (n) is logically related to act (1) via a chain of minimally deviant acts, proponents of this argument conclude that act (1) is already impermissible. Note that conceptual slippery slope arguments may fail because the minimal differences between the acts in question are shown to be ethically relevant: this would reveal that only the introduction of act (1) is justified, but not the introduction of acts (2) to (n).  

Whether or not a slippery slope argument has the theoretical shortcomings outlined must be examined on a case-by-case basis.

II. Slippery Slope Argumentation in the Case of Research Cloning

One conceptual slippery slope argument used against the permissibility of research cloning, the so-called continuity argument, states that there is no morally significant difference between embryos in immediately successive stages of development. Therefore, if the use of embryonic stem cells is permitted, the killing of older embryos (and also of humans already born) must also be permitted. This, however, is obviously ethically impermissible, so that the use of human embryos is also impermissible. The success of this conceptual argument depends, among other things, on whether there are ethically significant differences between embryos in successive stages of development. Suggestions for ethically relevant cuts are: Nidation, loss of totipotency, formation of the primitive streak, development of the neural tube, formation of the brain, and emergence of sentience.

In the form of a causal slippery slope argument, objections against the legal approval of research cloning are sometimes based on the conviction that the use of cloning technology for research purposes will make the misuse of the technology for reproductive purposes more likely. Critics of research cloning point to the technical similarities between therapeutic and reproductive cloning. However, it is difficult to identify concrete probabilities for such abuses. Whatever probability values one may assume, it is necessary to weigh different aspects: Are the risks of abuse of research cloning so great compared to the benefits of research cloning that research cloning should be prohibited by law?