I am interested in the slippery slope. Must I accept that the first instance or "slope event" that gives rise to the argument is in itself without much consequence? Or, can I argue slippery slope AND insist that the first instance (developing a parcel of public land, for example, that will result eventually in all the virgin land's demise) is a mistake?

A slippery slope argument in ethics typically has the following form:

If we were to deviate from the status quo in which X is disallowed and instead allow for X, allowing for X (which need not itself be morally objectionable or worrisome) will lead to Y, which is morally objectionable or worrisome. Therefore, we should not deviate from the status quo and allow X.

You ask whether the proponent of a slippery slope argument must hold that the first instance is "without much consequence" or if they can instead see it as a "mistake". As the above form indicates, slippery slope arguments generally assume that the deviation from the status quo that 'sets in motion' the slippery slope is not in itself objectionable or worrisome. Dialectically, the point of slippery slope arguments is to concede to one's opponents that the reform in question is not morally objectionable or worrisome, but argue that we ought not to pursue because doing so will lead us down a slippery slope to an objectionable or worrisome state of affairs. Note that this puts both parties to the dispute on common ground with respect to the intrinsic objectionability or worrisomeness of X, i.e., they both agree that X itself is not objectionable or worrisome.

That said, the common ground established for argumentative purposes does not preclude the proponent of the argument from thinking that the deviation in which X is allowed is objectionable or worrisome -- that it too is a mistake, independent of its alleged role in bringing about the objectionable slippery slope. The proponent might accept X 'for the sake of argument' without in fact believing it acceptable.

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