In this first half of the book, we are going to examine various factors that might be thought to possess moral relevance at the normative level. But the list of potential "candidate" factors is virtually unlimited; how should we decide which ones to discuss? My suggestion is quite simple. Most of us (the readers of this book, at any rate) share a common moral outlook which we might call commonsense morality. People may differ about the details, but at least the broad features are familiar and widely accepted. Even those people who reject commonsense morality -- whether in whole or in part -- are typically quite familiar with it (it may influence their moral intuitions, for example, even if these are intuitions that, on reflection, they are prepared to disavow). I implicitly appealed to commonsense morality in the example of the drowning woman (in 1.4), when I was trying to give intuitively plausible examples of morally relevant factors. I was counting on your acceptance of commonsense morality -- or, at least, your familiarity with it. Since commonsense morality recognizes and endorses the four factors that I mentioned, I assumed that you would either accept these factors yourself or else recognize them as factors that many other people regard as relevant.
This feature seems to me to be a desirable one when selecting factors for more extended examination as well: since we will obviously have to limit our discussion, it makes sense to select factors that we will all recognize as being widely accepted. So I propose to let the common moral outlook continue to be our guide, at least so far as setting the agenda goes: the factors that we will discuss in Part One will be those that are recognized by commonsense morality. Note that we can do this without necessarily endorsing the views of commonsense morality. We are only letting commonsense morality set the agenda for discussion; we are not assuming that all of the factors it recognizes are of genuine moral relevance.