Temporal moral relativism is the view that some moral rules are binding at one time but not other times. For example, some people think that adultery was morally wrong in 1965 and adultery is not morally wrong in 2016. The view is a denial of the existence of universal moral rules. Universal moral rules are rules that are binding for all people at all times in all circumstances. Thorsten J. Pattberg gives a reason why we might be tempted to hold such a view:
Certain forms of moral corruption that were once considered despicably evil are now acceptable behavior already; we don’t even think about them as moral failings at all. In the US, lobbying for congress is normal, which is essentially buying politicians, posts, and lucrative contracts. And while taking bribes is still punishable, giving bribes is mostly not. Homosexuality was long thought of as the breakdown of public morality; now it’s perfectly fine; so are promiscuity, divorces, abortions, and kids born out of wedlock. Pedophilia, polygamy, and sodomy are common aberrations of human behavior, with (possibly) biological components. We arbitrarily lock such people up, this may well continue, but who are we to call them “immoral?”The argument appeals to the historical fact that some moral rules that were once considered universal are not now thought to be binding. If past moral rules have been overridden, then present moral rules will probably be overridden in the future. Therefore, there are no universal moral rules.
The most pressing difficulty with such a view is that it assumes the following moral rule: 'it is right to obey your culture's moral rules at the time in which you live.' But this is self defeating. That moral rule is surely subject to itself; it can be overridden at a later time. If not, then there is at least one universal moral law and temporal moral relativism is false.
Moreover, for what reason should anyone accept such a rule? Why should everyone accept a culture's judgment on human actions? There appear to better options not least of which are ethical principles that when applied to human action inevitably lead to objective universal standards of action. 'Treat others as you would have them treat you' appears pretty sturdy, not culture, person, or time relative. Of course, if you, like I, believe there is a God, then God's prescription for human actions as he has revealed them in scripture are not subject to our whim depending on our culture or temporal location.
However, temporal moral relativists do not generally wish to get rid of all ethical reflection. They commonly suggest that it is the job of communities of people to decide on how to live with one another in order to further their flourishing. Just because rules change over time doesn't mean there are no guiding principles. For example, the harm principle can underwrite various rules adopted by different people at different times dependent upon cultural desires and context. In previous cultures monogamous marriage may have been considered the best way to achieve longevity and prosperity, but now, with the advance of technology and food production, and contraception, it is no longer necessary. However, the principle of harm restrains people from acting in such a way as to harm another person (by abandoning a spouse in previous times or by failing to use contraception in present times).
The temporal moral relativist may seek to modify her view by saying that moral rules change over time while moral principles stay the same.
What I would like to suggest is that the rejection of universal moral rules entails the rejection of moral principles and this, in turn, entails the rejection of ethics in general. In sum, I shall argue:
Either there is no ethics at all or there are universal moral rules
To say that there is no ethics is to say that there is no answer to any of the following questions: What makes a person good or bad? What makes an action good or bad? When should we think about ethics? If there are no answers to these questions, then there are no truths about the world that would provide answers. There is no property of persons in virtue of which anyone could judge them to be good or bad; likewise for actions. Furthermore, there are no circumstances in which it is appropriate to engage in ethical reflection, the kind of activity I am now doing.
The first thing to say is that there is no rules without principles. An ethical principle is some universal principle in virtue of which rules are produced. If an ethical principle says that a person should never treat another person as a means to an end, then a moral rule might be something like: 'It is wrong for combatants to use innocent civilians as a human shield to defend themselves from their enemies.' It is possible that there be ethical principles and no rules. The principles may be unknown or knowingly disregarded. Rules require mental labor, figuring out how some principle applies to a given group of people in some set of circumstance with some purpose. So, if there are principles and no one wants to do the work, then the principles lie dormant.
Those who think the rules change over time but the principles don't change think that mental labor produces different rules at different times and that the rules can (and often are) be in contradiction with one another: Some action at some time is morally wrong but at another time it is morally right (or permissible). What makes such a change justifiable is that the context of application is different. When one applies a principle at time 1 it produces rule 1, but when one applies the same principle at time 2 it produces rule 2 and rule 2 entails not-rule 1.
Either 'one should always obey the rules of a culture at a particular time (i.e the time at which you live)' is a principle or it is a rule. If it is a rule, then it is subject to its own defeat. At any time a culture could decide to change it, swap it out for its opposite. If it is a principle, then it is universal. But again, there is no good reason to accept it. Why think, for example, that white supremacist culture should be obeyed at a given time or Stalinist Kremlin culture should be obeyed at another given time? Moreover, if we were to accept such a rule it is hard to see how we would be able to change the rules. Surely if it is right to obey some culture at some time, then, if we are to apply the principle universally, we would not be able to stop obeying the rule!
One final point. Certain moral rules are not merely based on moral intuitions plus situations. They are based on proportions about the world that are non-moral but also universal.
Consider abortion. The moral principles in question are something like the following: (1) People have the right to choose what to do with their own bodies (2) We should other human beings the way we would like them to treat us.
On one view, an abortion is a part of the woman's body and therefore is not an innocent human life. How one goes about discerning whether or not an abortion is morally justified is based on whether the entity in the womb is a body part or a human life.
And here's the problem: The proposition, "the status of the entity in the womb is a human life a n weeks but not a human life before" is not the kind of fact that can change over time. In other words, if the developing fetus gains its status as human life at a certain point in pregnancy in 2016, then a developing fetus gained its status as human life at the same point in a pregnancy in 1900. If so, then either the principle (2) that we shouldn't willfully destroy a human life is false (or subject to change over time) or abortion after that point in time is morally wrong at all times.
The point is that moral principles are related to truths about the world. And some of the truths about the world to which they relate don't change over time.
So, moral relativism (of any kind) is self-defeating. However, as Peter Kreeft points out, moral relativism is less like an argument and more like a disease. It sounds like a rational way to think but is actually merely a denial of reasoning. What's the cure? According to Kreeft, a call to repent, fast, and pray. For only God can cure such an ill.
[CAVEAT] A caveat is due at this point: there is a difference between a moral rule being absolute and it being objective. For a moral rule to be objective it must be something in the world and not merely subjective to our whim. For it to be absolute means that a given moral law cannot be overridden by any other consideration. Temporal moral relativism denies the objectivity of moral rules not merely the absolute nature of moral rules. Of course, it is possible to accept the objectivity of a moral rule without thinking of it as absolute (see here for a good explanation).