The life of everyone is equally valuable as the life of everyone else, other thigns being equal. Nobody would reject this proposition. But is it really true? Imagine that you are faced with a choice between transplanting a new heart to a 20-year-old patient and transplanting it to a 60-year-old patient. What does the above proposition imply? The answer depends on how we interpret "other things being equal". If the number of years lived (or the number of years to be lived) is morally relevant, there is a good reason to give the heart to the 20-year-old. Otherwise, we should flip a fair coin. However, what "other things being equal" implies is not stated clearly.
General problem: Can this uncontrovertial proposition be used as the besis for a moral view? Many philosophers use a seemingly self-evidently true proposition to state their moral view, not to argue for their moral view. I call their strategy circularist. I belive many central concepts in moral philosophy (e.g. agency, the separateness of persons, etc) are used to state their moral view, although these concepts are supposed to ground their moral view. In this project, I will elucidate the problems of circularism in various contexts.