We tend to think that our moral obligations fall off with distance: the closer someone is to us the greater is his moral claim on us, and the further away the less. Morality operates like gravity—it weakens with distance. True, morality is an expanding circle, but it is also a diminishing sphere. At the outer limits it hardly gets a grip at all. It would be a mistake to interpret this notion of distance as mere spatial separation, though that seems to be one component of it. In addition to distance in space there is also distance in time: the more temporally remote some future person is the less hold she seems to have on us morally. What obligations do I have to people in the 30thcentury? If I have any, they are not so strong as the obligations I have to people now. Whether this is rational or morally justifiable is a debatable question, but as a descriptive truth about our moral attitudes it is surely correct. Further, there is the dimension of personal contact or emotional proximity: the more intimate my relationship with someone the greater the obligation I feel. This applies to family, spouse, friends, colleagues, and so on. It may be that spatiotemporal distance is really just correlated with this dimension, which is the underlying factor; relationship-distance is the main consideration. We might also add psychological similarity: we tend to regard beings similar to us psychologically as deserving more of our moral concern—humanlike, mammalian, warm-blooded, non-alien. Thus our contemporaneous close relatives have a stronger claim on us than a jellyfish-type creature living in a remote galaxy two million years from now. Moral distance is multi-factorial and complex not just a matter of physical miles. It introduces degrees of obligation into moral duty instead of just the all-or-nothing binary opposition of duty and non-duty. It also introduces uncertainty and messiness into our moral calculations.
It is helpful to picture the diminishing moral sphere as follows. At the center lies the ever-present self: this is the being minimally distant from the moral agent (they are identical) and it has a uniquely strong influence over our decisions. Given that prudence is also moral concern for one sentient being among others—I am a valuable being just like everyone else—we can think of prudence as the basic case of moral obligation. I am obliged to be concerned about my own interests and I am extremely close to myself. I am the center of the sphere of my concerns and others radiate outwards from me. The next closest being is then a matter of individual variation: it could be my spouse or my parents or my children or my friends, depending on circumstances. Then we get to the much more extensive circle of my general acquaintance. After that we have members of my local community perhaps; then other countries; then other species; then the next generation; then more remote generations; then beings in other galaxies; finally completely alien life-forms in remote regions of space millennia hence. My own interests come first (other things being equal and given a degree of selfishness) and then the interests of others according to their place in the sphere. This is the whole sphere of my moral obligations, and it varies in degree of demandingness. Most obviously, there is variation in the strength with which I am obliged to reduce or prevent suffering.
Imagine if someone inverted this ordering of moral priorities: she treats the more remote beings as having a stronger hold on her moral concern. Creatures in the distant future on remote planets that are psychologically dissimilar to her occupy the center of her moral universe, while family and friends have merely marginal moral interest for her (she might even regard herself as morally negligible). That would certainly strike us as bizarre, insane even, but it is not easy to see how we could persuade her that it is irrational or immoral (she might point out that they are suffering sentient beings too, equally deserving of respect and care). But by the same token it is hard to see how we could be deemed irrational for our ordering. Nor does it seem justifiable to insist that only equalconsideration is rational or moral, so that we must treat spatiotemporally remote beings as morally interchangeable with our nearest and dearest. In fact, the distribution that seems the most natural is precisely the one that we adopt—despite the fact that no obvious foundation for it can be produced. Perhaps it is just psychologically necessary for human beings or other evolved creatures, or even for all beings with emotions directed at others: no other moral psychology is feasible given the basic nature of sentient beings. Ought implies can, so there is no point in reprimanding us for favoring the more proximate beings. Not that we can or should have no concern for the remote and alien, but it must of psychological necessity be diluted and relatively undemanding. If this means that we cannot occupy an entirely impartial and objective moral perspective, then so be it; at least the perspective we have is workable and not too destructive or callous. Brain surgery that changed our moral psychology so as not to discriminate against the distant and different might totally wreck everything that makes human life worthwhile, or even possible. How could you marry someone who systematically favored the remote over the proximate? What would happen to loyalty, trust, solidarity, etc.? What would happen to family life if parents treated every child in the world as deserving the same care and attention as their own?
It is not an easy matter deciding how robustly moral obligation extends to the distant objects of possible concern. Morality has not evolved with these quandaries in mind. For instance, we have not had to think about how our actions will affect the wellbeing of people in future generations, as in climate change; nor did our ancestors put much thought into our obligations towards animals. I don’t want to argue that our current distribution of moral concern is correct and beyond reproach, only that it is not irrational to treat distance (in the multi-factorial sense) as morally relevant (certainly we should be careful about trying to reconfigure our psychology to adopt a more impartial point of view). I would not, for example, be happy to see support for foreign aid curtailed in favor of a supposed more pressing need on the part of future generations, or the political plight of the Venusians. This is also not a point about favoring humans over non-humans: I am all for treating our own animals as having moral priority over more distant animals, because this exhibits the kind of relational closeness that confers moral priority (though I also think remote animals do deserve some moral consideration). Some sort of moral ordering seems inescapable, but whether we have it right now is another question. What we should not do is try to motivate concern for our fellow man (and other animals) by appeal to some perfectly general principle banning all forms of moral distancing, as if every sentient being in the universe had an exactly equal claim over us.  Things are more nuanced than that, and more difficult to resolve.
 People sometimes say that we should try to occupy a God’s-eye view of creation, morally speaking, treating all sentient beings equally. But God does not exist in space and time, and he has no selective emotional relations with human beings and other animals. We do, and it is folly to try to make us take up a Godlike moral perspective. We can take this perspective into account, but we shouldn’t be governed by it, on pain of possible psychological collapse. In any case, there is no demonstration that the diminishing sphere that we habitually operate with is irrational or immoral.