Here is a typical argument with an unstated premise:
The use of condoms is completely unnatural. They have been manufactured for the explicit purpose of interfering with the natural process of procreation. Therefore, the use of condoms should be banned.
In this argument, the first two sentences con- stitute a single premise, the gist of which is that using condoms is unnatural. The conclusion is that the use of condoms should be banned. This conclusion, however, does not follow from the stated premise. There is a logical gap between premise and conclusion. The argument will work only if the missing premise is supplied. Here’s a good possibility: “Anything that interferes with a natural process should not be allowed.” The argu- ment then becomes:
The use of condoms is completely unnatural. They have been manufactured for the explicit purpose of interfering with the natural process of procreation. Anything that interferes with a natural process should not be allowed. Therefore, the use of con- doms should be banned.
By adding the implicit premise, we have filled out the argument, making it valid and a little less mysterious. But now that the missing premise has been brought out into the open, we can see that it is dubious or, at least, controversial. Should everything that interferes with a natural process be banned? If so, we would have to ban antibiotics, anticancer drugs, deodorants, and automobiles.
(Later in this chapter, ways to judge the truth of moral premises are discussed.)
When you evaluate an argument, you should try to explicitly state any implied premise (or premises) when (1) there seems to be a logical gap between premises or between premises and the conclusion and (2) the missing material is not a commonsense assumption. In general, the sup- plied premise should make the argument valid (when the argument is supposed to be deductive) or strong (when the argument is supposed to be inductive). It should also be plausible (as close to the truth as possible) and fitting (coinciding with what you think is the author’s intent). The point of these stipulations is that when you supply a missing premise, you should be fair and honest, expressing it in such a way that the argument is as solid as possible and in keeping with the author’s purpose. Adding a premise that renders an argu- ment ridiculous is easy, and so is distorting the author’s intent—and with neither tack are you likely to learn anything or uncover the truth.
Be aware, though, that some arguments are irredeemably bad, and no supplied premise that is properly made can save them. They cannot be turned into good arguments without altering them beyond recognition or original intent. You need not take these arguments seriously, and the responsibil- ity of recasting them lies with those who offer them.