Is morality objective?

In practice, morality varies from place to place and time to time. But should it? Is there some universal moral code which applies to all peoples in all places, or is custom king?

Definitions

 * Morality is defined as "a doctrine or system of moral conduct".
 * Moral is defined as "relating to principles of right and wrong in behavior".
 * Objective is defined as "relating to, or being an object, phenomenon, or condition in the realm of sensible experience independent of individual thought and perceptible by all observers; having reality independent of the mind".

Pro

 * Despite some differences in moral practice, there are substantial similarities in what many cultures think should be correct behavior and there are also some taboos which are very common in practice. For example, the prohibition of murder or marriage between siblings. These can form a basis for a cosmopolitan morality.
 * A cosmopolitan morality is not an objective morality. Objectivity is completely independent of sentient beings, by definition.
 * Humans are about 99.9% genetically similar to each other and may have evolved to feel some acts as repulsive and others as good. Thus, there may be other explanations for similarities in our moral practice and we cannot argue that coming to the same conclusions implies that we are understanding an objective moral reality.
 * If morality were objective, one would expect all of it to be objective, not just the part which most human cultures tend to agree on. How could "is murder bad?" have an objective answer while "is premarital sex bad?" does not?
 * Objective morals are neutrally vouched for by the many, subjective personally by the individual; we'd expect the spectrum of objective-subjective to somewhat comply with a spectrum of communal-personal identification. Humans share a default interest to live, and to do so in good quantity (i.e longevity) and quality (i.e harmony). Any objection to do some arises from an individually motivating reason derived from an individually constructed set of beliefs deviating from the ideal (suicide is beyond this argument's scope, so I digress). Whether two people wish to engage in premarital sex is of no comparable threat to a community to an act of murder, the latter of which appeals to our primal instincts to defend ourselves and our loved ones, and survive in doing so. What human cultures come to agree on then have some root in instinct and hardwired imperative, for we are, after all, equally weak as we are potentially destructive.
 * Such is a false standard. There are substantial similarities, but we did not say equalities. Humans are dumb at times.
 * Human morality is based on human psychology which is an objective part of the Universe. Therefore, morality is an objective part of the Universe.
 * While neuroscience and brain chemistry do represent a more objective part of psychology, the internal psychology that morals are built upon come from our subjective experience of what we have learnt or been taught is right or wrong. If we look thousands of years ago or at different cultures that normalized acts of crime that we in the modern day feel are morally wrong, they had those views of morality, contradictory to our own, because they experienced a different environment that taught them to think that way.
 * Decisions based on human psychology are by definition subjective, thus, morality is not objective.
 * Just because you think you are a rat doesn't make you one, human psychology is objective for that reason
 * Just because you think you are a rat doesn't make you one implicates, that thinking something is moral or immoral have the same ammout of objective truth, therefore not objective.
 * That does not explain why assessment of morality varies between cultures and subcultures. Since, "based on human psychology" sounds as if morality is innate, and it is unclear what else is that supposed to mean. And furthermore, if morality is innate, it is possibly as much subject of variation as color of the eyes. Thus, the above argument has almost no force.
 * Objective morality exists under the guise of modern game theory.
 * Game theory describes strategic decision making, not which decisions are morally wrong or right, objective or subjective. It would be needed to claim that acting rationally is good while acting irrationally is bad to call game theory a morality. And that would of course be a subjective claim. Rationality does not call itself morally good.
 * We can arrive at an objective ethical norm by universalizing the duty that exists intrinsically in interests: (1) "My interests should not be frustrated"; (2) "The interests should not be frustrated"; (3) We add the consideration of the omissions: "A priori, the interests should not be frustrated".
 * These premises are unproven and there's no objective reason to accept them.
 * Morality is promoting happiness, well-being and health, while immorality is promoting harm, suffering and pain. All of these phenomena are rooted in objective, measurable biological processes. Therefore, morality is objective.
 * Pain aversion doesn't define morality as objective, that's using an evolutionary mechanism as a definition of morality, which causes it to fail. For example, else a lot of arbitrary daily activities would be deemed immoral, like working at a job you are 'unhappy' at. Or, sexual activities are pleasurable to most people but yet deemed immoral in a lot of religions outside certain context.
 * Suppose the objective existence of a/the perfect God is proposed: then morality can be defined relative to God's "character": the closer aligned an action or being is to God, the more moral. This is an objective stance regardless of the subjective interpretations of said morality. Any argument for a particular God's existence hence implies moral objectivity's existence.
 * Even if we suppose the existence of a God, all morality would be the subjective opinion of that God. You would still be left with the following question: Why is it moral to follow God’s nature?
 * Plato piety problem. A perfect God would know what is right because it is right, and not because it just says so.
 * If we suppose the existence of a God, all morality would be subjective to that opinion. However, that opinion would be right, as it defines what is right. That once subjective opinion would become the objective standard as it is the origin of all things. (supposing the existence of a God, of course) All things come from that God, thereby,  all moral standards come from that God, if you have a moral standard that goes against, then you are objectively wrong.

Con

 * Philosophers, religion reformers, and legal theorists have argued for millennia about what objective morality should be, but they haven't come to a conclusion yet.
 * This argument entails the Problem of induction. "This question has never been definitively answered in all of human history" was once true for every question that has been answered definitively by philosophy, logic, mathematics or science.
 * If a conclusion has not yet been reached, we can't assume that one will be reached or that an answer exists.
 * It is hard, if not impossible, to find any moral issue on which every culture agrees. If morality were objective, we could expect to have at least some basic agreement, like with basic chemistry. But we don't have such agreement, so morality must be subjective, or intersubjective, but in any case not objective.
 * This argument aims to ground ethics in existing morals. All existing morals could be wrong. Other people defend that ethics is based on reason, independently of existing morals.
 * Although there are differences in moral practice among cultures, that doesn't imply that there should be differences (see is–ought problem).
 * Cultures do not agree that God does not exist, and yet he objectively does not. For very long time, those who correctly determined God not to exist were a minority. Whether cultures agree on something or not has little bearing on objective reality.
 * There is no clear evidence that God does exist
 * The matter of which is applied is always subjective. Any standard deemed objective, if applied to moral, would surely give rise to an objective moral standard. But surely other equally objective standards can be applied, so there is no single objective standard.
 * If there is an objective set of moral rules, then no set of rules that contradicts said rules can be objective since such a set would be wrong. Any set of rules that complements said objective moral standard without contradicting it could just be included in that moral standard. Thus, if morality is objective, then there are no sets of objective rules that meaningfully differ from each other; there is simply one set of rules.
 * This sounds like an argument for subjective morality rather than a relevant objection. There is no one list that can be proved to be the objective list, since a rule’s membership is categorically contingent on subjective standards.
 * The question "Why should I act morally?" cannot be answered. If it is answered along the lines of "because it is the nature of morality" then you are justifying morality with morality, which is circular logic. It is like a law saying "The law must be obeyed". If someone don't care about the law then they won't care about the law that tells people to adhere to the law. If it is answered with anything other than morality itself then that reason is a ulterior motive, and needing ulterior motives to act morally goes against the definition of morality.
 * This doesn't actually answer the question. "Why should I act morally?" Is a different question from "what is moral?", or "is morality objective?". In the same vein (to use the stated analogy), "why should I follow the law?", it is a different question than "what is the law?", or "is the law objective?". Notice, the first question does not have an answer, yet laws can be objective. Therefore, morality can be objective even if there is no reason to be moral.
 * Morality, as defined by humans, is meant to determine the goodness or badness of an action or intention. To consider if an action is good or bad objectively, then there must be some objective end-goal so that we can measure the extent to which this action pushes towards or away from it. Humans may have very similar end-goals baked into our biology related to species survival, but that does not constitute a universal end-goal. Even if someone were to make the claim that the universal end-goal is space expansion, or any other claim, there is no possible proof for these claims.
 * If morality is objective then it is not defined by humans; this is a circular assumption of subjectivity that demonstrates itself.
 * The fact that we are not aware of a universal end-goal (or choose to reject well known alternatives, eg. on the basis of religious belief) does not disqualify the possibility that such a concept exists. Even if there is no available proof for such a goal's existence, nor is there for its falsehood.
 * As for "there is no possible proof for these claims": the question is whether morality is objective, meaning whether it objectively exists, not whether it can be known or proven to be such and such.
 * Nothing is objective. We must first define a concept before we are able to observe it in the world. There is no single way to define a concept that does not exist until we define it. For morality to be objective, it must, by definition, be true uniquely a priori. Knowledge of objective moral principles, if such knowledge can even be considered objective in itself, would necessarily result in the application of such objective moral principles in relation to experience in the external world, since the inherent purpose of morality is its application in actuality.
 * The claim that nothing is objective is unfalsifiable and not provable. However, the claim that objectivity is not real is in itself objective, showing itself to be paradoxical.
 * We define concepts based on our observations of the world, not vice versa as "Nothing is objective" claims. We define concepts so that they can be identified by the human race as they are observed in the world.
 * The conflation of the essence of a moral principle a posteriori (as understood by humans) and the principle itself a priori is invalid: an interpretation of an objective morality by a subjective person may be subjective, but it is also independent of that same morality.
 * It is a falsehood to claim that we can derive the objective moral principles from their application to the world, since this application is indeed of a subjective interpretation.
 * Imagine I make up a concept, "blorg". Can science automatically denote how much "blorg" one thing has compared to another? Not without a definition of "blorg". Can science create a definition for "blorg"? No. If I make up a universal definition for "blorg", then science just comes after to apply that definition to situations. But science does not figure out what "blorg" is. Similarly, we need to decide what morality is before we can use math to apply morality to situations. If we decide what morality is, then it is not objective.
 * The final statement of this argument is its very disproof: this argument assumes that the definition of morality is sourced from humans, which is not provable.
 * According to definitions given above, the statement "Morality is objective," may be restated as "There exists a set of principles of right and wrong behavior which is independent of individual thought and perceptible by all observers". If such a set of principles exists, that set is perceptible to all of this debate's participants. No such set is observed by all participants; thus the affirmative position is that a set of principles of right and wrong exists and is perceptible to all observers but is not perceived by all observers. If this set of principles has never yet been perceived, that set can not be differentiated empirically from a set that is nonexistent. If this set of principles has been perceived by an observer, that observer need only to present the set to others in order to prove the affirmative (by definition, the set of principles of right and wrong will be perceptible to any new observer and their correctness independent of the observer's thought). No such set has been presented.
 * Such sets have been presented by observers. When these sets are under debate, often the debate does not focus on whether the guiding principles are right or wrong but on how the guiding principles apply to a given situation. For example, the question, "is it wrong for a starving child to steal expired food from a store that is out of business?" may be debated from many different angles and different people may arrive at many different conclusions, but the debaters would likely all agree on certain principles (children starving is bad, stealing is wrong, expired food is less valuable, out-of-business stores are less capable of experiencing the loss caused by stealing) and the debate would center around which of these moral principles should weigh more heavily in this situation.
 * This objection attempts to present such a set of principles of right and wrong behavior whose truth is perceptible by all observers, such as ["children starving is bad," and "stealing is wrong."] The existence of a single observer to disagree with the items in such a set will undermine the argument that the set is objectively true. There are observers to argue each point in any such set, such as the parent who believes it is moral to starve a child for disciplinary purposes, and the thief who feels confident in the morality of their behavior. We might make infinite attempts at refining such a set, but will always easily find an observer who does not agree with it. If morality is an objective truth, independent of the mind, perceptible to all observers, there is no justification for the one true morality being impossible to agree upon.
 * This argument makes the assumption that the "set of principles...is...perceptible by all observers" and argues that because no such set of principles exists, there is no morality. However, the premise that a set of principles is always equally perceptible to all observers is not required by objective morality. In the same way that gravity always exists but will be more tangible experienced by someone on the ground than someone floating through space, if objective morality exists, it may not necessarily be equally clearly perceptible to all observers at all times.
 * The sourced definition of "objective" includes being "perceptible by all observers." If gravity were not equally perceptible by all observers, there would exist an observer who could not perceive gravity, and thus gravity could be said to not objectively exist, for this observer would be the only necessary proof that gravity is a phenomenon that is not experienced by all observers. Gravity does meet this requirement for being objectively true, despite the fact that its effects can be difficult to perceive, just as the Sun's existence is an objective truth even at night.
 * If we use the same reasoning, we cannot state definitively that the Earth is round, as some observers still claim it is flat
 * The definition of "objective" requires the thing to be equally perceptible to all observers. The Earth can be said to be objectively round, however the existence of "flat-Earthers" and debate on this matter indicates that Earth's roundness is not perceptible to all observers. Nevertheless, the Earth is objectively round. The definition of "objective" being used is flawed.
 * The sourced definition of "objective" is not flawed. Flat-Earthers may not perceive the fact that the Earth is round. The roundness of the Earth is, however, perceptible to them. All of their daily perceptions are of a round of Earth, all of their experiences on Earth are explained and dictated by a round Earth. There is no reproducible experience or experiment among all observers on Earth that denies the roundness of the Earth, because it is an objective truth, perceptible to all, independent of mind.
 * There are humans who objectively lived, but who left literally zero evidence of their lives. By the definition being used, these humans did not objectively live. The definition is flawed.
 * It is objectively true that there have been humans who left zero evidence of their lives. It is objectively true that such historically unknowable persons existed. Since these unknowable persons left no evidence of their existence, nothing else can be objectively stated about any such individuals. Such unknowable persons objectively lived, but no such individual can be said to have lived objectively. The definition of "objective" still asserts the correct objectivity of these facts.
 * The argument admits "unknowable persons objectively lived", this disproves the original definition that for something to be objective, it must be capable of being perceived

Morality is semi-objective

 * Morality is sometimes objective and sometimes not. Objectivity in this context does not refer to inerrancy but to the description of whether the moral suggestion made can be independently reproduced and agreed with by at least two people, versus a subjective morality which would be incommunicable -this is often the case with implicit core beliefs about morality that people know not to say aloud-. There are multiple objective moral systems, so long as they are reducible to first principles and are expressible.
 * If there are multiple moral systems, they either perfectly align and form a single objective moral system, or they conflict and thus none is an objectively true system of morality.