Someone told me that the idea that there's no objective truth in ethics is highly debatable. I am wondering what are some purported objective truths identified or claimed by various philosophers throughout the years. I am thinking there might be one or two extremely simple assertions that could be objectively true, but I can't really think of any of them.
The controversial issue isn't whether there are some objective truths in ethics, but whether statements about ethics are the kinds of statements that can be objectively true at all. This is an old and well-travelled problem in ethics. A moral realist would say that moral truths are just what they seem to be. If I promise to pay you $20 then I have an obligation to pay you $20, and if I fail to do so, I have done something objectively wrong.
Notice that I used ethics-related terms to describe ethics: "obligation" and "wrong". How do I define what ethical behavior is without using ethics-related words? Most moral realists would say that you can't. The class of ethical "things" like obligations and wrongness is a different category of thing from anything else. They aren't physical objects, they aren't thoughts, and they aren't abstract objects, nor can they be reduced to any of these things. Consequently, you can only describe ethical terms in ethical terms, much like you can only describe physical terms in physical terms and abstract terms in abstract terms. For example, if you are trying to describe where Dallas is, you have to refer to other physical things such as Texas, or at least the earth.
There are two big problems with moral realism. First is the status of these moral objects. What kinds of things are they and how do we know about them? The other, related problem is that there is no agreement on what is moral and what is not. If people can't agree on an object, one obvious implication is that the object is not real. However, every alternative to moral realism has other implications that are also undesireable.
The other approaches to ethics all try to reduce statements about ethical "things" to statements about some other sort of things. Epicureanists believe that moral good is equivalent to pleasure. In other words, they reduce "X is good" to something like "X gives me pleasure" or, if they want to try to construct something similar to traditional morality, they might reduce it to "X is good because it provides the most pleasure for the most people". The problem with this approach is that it has unfortunate moral consequences; for example, it is good to torture a man to death if watching the torture is entertaining for enough people that their pleasure outweighs his pain.
Moral relativists reduce ethics to something like laws or conventions. In other words, what is morally good for you to do is based on your upbringing or the society in which you live. This theory also has unfortunate moral consequences. According to theories like this, the Aztec priests who ripped children away from their weeping parents and sacrificed them to the gods were doing something morally good.
Moral pragmatism reduces morality to effectiveness or practicality. In one version of this, moral codes are designed to optimize the safety and interactions of everyone in society. For example, trade can only work if people keep their word, so you have an obligation to keep your word. The problem with this version is that it doesn't make it immoral to cheat, so long as you don't get caught, so moral obligations only matter when other people are watching.
The evolutionary approach to morality is the idea that morals are just how we have evolved to interact with each other, that there is some evolutionary advantage to behaving morally. The problem with this approach is that it makes genocide moral.
The only one of these theories that can allow for objective obligations is the realist theory because all of the others reduce "X is wrong" to something else:
Epicurianism: X is not the best way to experience pleasure.
Relativism: X is unconventional.
Pragmatism: If everyone did X, society would not work as well.
Evolution: X goes against evolutionary programming.
None of these is really the same as saying that you should not do X just because it is wrong. They all give some other reason to not do X. So when someone says that a moral judgment is objective and real, they are appealing to moral realism.
There are various other approaches and an enormous literature on the topic. If you are interested, I suggest you start with the Stanford Encyclopedia of Philosophy, which is a tremendous online resource.
Ill-formed question: ethics does not define any truth.
To start, ethics is a system of rules, that is, a coherent set of statements that rule behavior. For example, do not kill, help the suffering, etc. System means a set of interrelated parts; ethics is a system because it defines rules at multiple levels: ideals, behavior, social interaction, interaction with God, etc.
Notice that following the definition, ethics is a COHERENT set of rules, this implies the use of logic, but not to relate rules with the truth (that is, it is not with the intention to say "this rule is true"); it is only to follow a coherent behavior. For example, if the first rule would be "kill", then, both rules would be contradictory, because you would waste your time trying to kill people and at the same time, trying to make them survive.
So, how do we know, for example, if "ethics is good"?, which is a question that CAN be associated with an ideally "objective" truth (objectivity is normally considered a shared agreement)?
Ethics per se does not target "good", or "altruism", or "survival". What connects ethics with truth are ethical theories: utilitarianism, kantianism, aristotelianism, etc. For example, utilitarianism states that the choice that provides good to most people is the right one. So, if you should decide between "kill" and "don't kill", you evidently will choose the second.
Regarding the meaning of good, that is another topic, and that's not related only with ethics, but to any system of rules. The theory that I consider most valid is the one that states that good is related to the survival of the group (read once in some phil. paper), that is, that systems of rules target survival.
So, if a gang of drug traffickers are confronted to the decision of killing someone if the group risks its existence, they will do so. Evidently, there's another system of rules that take more people into account: state law & justice. So, if justice notices that the lives of society members are in danger due to the group of traffickers, traffickers need to be killed.