In many ethical connections, including those in which the discussion concerns what the law should be, there is a well-known argument against allowing some practice, that it leads to a slippery slope. The argument is often applied to matters of medical practice. If X is allowed, the argument goes, then there will be a natural progression to Y; and since the argument is intended as an objection to X, Y is presumably agreed to be objectionable, while X is not (though of course it may be objectionable to the proponent of the argument – the slippery slope may be only one of his objections to it). The central question that needs to be asked about such arguments is what is meant by a ‘natural progression’. Before coming to that, however, we need to make one or two preliminary points. First, it is worth distinguishing two types of slippery-slope argument. The first type – the horrible result argument – objects, roughly speaking, to what is at the bottom of the slope. The second type objects to the fact that it is a slope: this may be called the arbitrary result argument.

An example of a horrible result argument is that sometimes used against in vitro fertilization of human ova. IVF gives rise to extra fertilized ova, and experimentation is at least permitted, and perhaps required, on those ova. The period of time during which such experiments are allowed is limited, but (the argument goes) there is a natural progression to longer and longer such periods being permitted, until we arrive at the horrible result of experimentation on developed embryos.