Gwern has an excelent article arguing against this thesis, which you should read in full. He argues that Singer ignores numerous cases in which our circle of concern seems to have shrunk. From the standpoint of our current ethics, it is hard to see these past concerns as bearing any moral significance, but this is always true of someone in a narrower circle judging someone in a wider one.
For example, our circle has narrowed with respect to gods:
When one doesn’t believe religion deals with real things at all, it’s hard to take religion seriously—much less recall any instances of its sway in the West increasing or decreasing.
But nevertheless, when one compares modern with ancient society, the religious differences are striking: almost every single supernatural entity (place, personage, or force) has been excluded from the circle of moral concern, where they used to be huge parts of the circle and one could almost say the entire circle.
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Iceland is mocked when construction is held up to expel elves—but the construction goes forward. Japan keeps its temples on sacred places—when they earn their keep and do not block housing projects, of course. Lip service is paid, at most.
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Peter Singer focuses on animals; religion gives us a perspective on them—what have they lost by none of them being connected to divinities and by becoming subject to modern factory farming and agriculture? If you could ask snakes, one of the most common sacred animals, what they made of the world over the last millennia, would they regard themselves as better or worse off for becoming merely animals in the expanded circle? If India abandoned Hinduism, what would happen to the cows? We may be proud of our legal protections for endangered or threatened species, but the medievals protected & acquitted ordinary bugs & rats in trials.
The circle has at best oscillated with regards abortion and infanticide:
Continuing the religious vein, many modern practices reflect a narrowing circle from some points of view: abortion and contraception come to mind. Abortion could be a good example for cyclical or random walk theses, as in many areas the moral status of abortion or infanticide has varied widely over recorded history, from normal to religiously mandated to banned to permitted again.
The use of torture comes and goes:
State use of torture can be cyclical—some northern European countries going from minimal torture under their indigenous governments to extensive torture under Roman dominion back to juries & financial punishments after Rome to torture again with the revival of Roman law by rising modern centralized states and then torture’s abandonment when those states modernized and liberalized even further. China has gone through even more cycles of judicial torture, with its dynastic cycle.
As well as other parts of the judicial system:
Some areas have changed far less than one might hope; arbitrary property confiscations that would make a medieval England freeman scarlet with anger are alive and well under the aegis of the War on Drugs, under the anodyne term “asset forfeiture” as a random form of taxation. (And what are we to make of the disappearance of jury trials in favor of plea bargaining?)
Our ancestors no longer command the respect they once did:
Another possible oversight is the way in which the dead and past are no longer taken into consideration. This is due in part to the expanding circle itself: if moral progress is indeed being made, and the weight of one’s voice is related to how moral one was, then it follows past people (by being immoral) may be ignored. We pay attention to Jefferson in part because he was partially moral, and we pay no attention to a Southern planter who was not even partially moral by our modern lights.
More dramatically, we dishonor our ancestors by neglecting their graves, by not offering any sacrifices or even performing any rituals, by forgetting their names (can you name your great-grandparents?), by selling off the family estate when we think the market has hit the peak, and so on.
Given these shrinking circles, should we call it an expanding circle or a shifting circle?
The expanding circle refers to the range of sentient beings that humans include in their scope of concern. The fact that we have recognized the non-existence of gods or the non-sentience of a fetus is not the same as shrinking or the circle. The fact that we no longer have concern for the dead is, again, not relevant. Our consideration for property is equally rejected by this simple consideration. You could certainly argue that from the perspective of a past human, the circle would seem to shift, but that does not change the fact that using our reasonable modern understanding of the world, the scope of human concern for sentient beings has consistently expanded, rather than shifted.
Gwern has an excelent article arguing against this thesis, which you should read in full. He argues that Singer ignores numerous cases in which our circle of concern seems to have shrunk. From the standpoint of our current ethics, it is hard to see these past concerns as bearing any moral significance, but this is always true of someone in a narrower circle judging someone in a wider one.
For example, our circle has narrowed with respect to gods:
The circle has at best oscillated with regards abortion and infanticide:
The use of torture comes and goes:
As well as other parts of the judicial system:
Our ancestors no longer command the respect they once did:
The expanding circle refers to the range of sentient beings that humans include in their scope of concern. The fact that we have recognized the non-existence of gods or the non-sentience of a fetus is not the same as shrinking or the circle. The fact that we no longer have concern for the dead is, again, not relevant. Our consideration for property is equally rejected by this simple consideration. You could certainly argue that from the perspective of a past human, the circle would seem to shift, but that does not change the fact that using our reasonable modern understanding of the world, the scope of human concern for sentient beings has consistently expanded, rather than shifted.