Rawls first draws the distinction between the thin and the full theories of goodness in §60 of A Theory of Justice. The first and perhaps the most important thing to note about the distinction is that it is a distinction between two theories or accounts of goodness. It should not be conflated, as it sometimes is, with a very different distinction that is drawn by some of Rawls’s critics. Communitarians sometimes oppose “thin” to “thick” and predicate these terms of conceptions of the good endorsed by members of Rawls’s well-ordered society or by members of the actual societies with which we are familiar. But this is not the distinction Rawls is drawing. He opposes “thin,” not to “thick,” but to “full” and he applies both adjectives to accounts of what makes something good rather than to conceptions of the good.

The thin and full theories differ in two respects. First, they differ in the resources on which they draw. The full theory “takes the principles of justice as already secured” (TJ 349) and so draws on those principles to account for goodness, while the thin theory does not. This difference leads to the second difference, which is one of scope. The thin theory can account for the value of some goods, such as the primary goods. Indeed, Rawls says that he introduces the thin theory precisely to account for the value of those goods (TJ 347), though we shall see later that the thin theory can account for the value of other goods as well. Because the full theory draws on more philosophical resources than the thin theory does, it can account for values that the thin theory cannot. In particular, because the full theory draws on the principles of justice, it – unlike the thin theory (see TJ 381) – can account for various kinds of moral goodness.