While it is easy to understand why somebody might say that we all know the difference between right and wrong, because most people do and because in general what is right causes most people to be happy and what is wrong causes suffering, the fact that some criminals either aren’t aware of their actions being wrong or don’t care suggests that this claim is not sound. 

Firstly, while St Thomas Aquinas did argue that we all have a God-given desire for the good and to avoid evil (Conscientia, the Primary Precept of Natural Law) he argued that we need to learn how to put this desire into practice by developing our Synderesis (what Aristotle called Practical Reason). Superficially it might seem that inherently wrong actions serve one or more of the basic human goods of natural law.  For example, killing my enemy might seem like it promotes peace and harmony in society.  Nevertheless, it also goes against the basic human good of life, so I must learn that killing can never be right, even when it affects an unpopular troublemaker!  When people do evil things, argues Aquinas, it is usually because they are pursuing an apparent and not a real good and this because their synderesis has not been properly formed.  For Aquinas then, it is not fair to say that we all have clear knowledge of the difference between right and wrong or that this is “just common sense”.  While we have good instincts for morality, we need to learn what is right and what is wrong and it is not always the way these seem superficially. 

Secondly, Immanuel Kant argued that the moral law appeals to peoples’ reason as a synthetic a priori, suggesting that we all have a knowledge of good and evil regardless of our experience in the world.  Nevertheless, he was at pains to explain how what is right is not always what makes people happy or seems instinctively to be the correct course of action.  Take, for example, Kant’s famous axe-murderer example from the Groundwork for the Metaphysics of Morals (1785).  Kant argued that it is right to tell the truth, even though we suspect that this will lead to the death of a friend, rather than tell what seems to be a white lie to save a life.  This is far from being common-sense!  For Kant though, reason shows us that if we can’t universalise the maxim of lying and would be using the suspected axe-murderer as a means to an end of saving our friend then what we do would be irrational and wrong.  The moral law is a categorical imperative and will not allow us to make decisions situationally, but demands consistent rational action. This shows that while we all have the ability to know the difference between right and wrong, this is not just common sense but requires deep thought and analysis. 

On the other hand, utilitarians like Jeremy Bentham argue that “nature has placed mankind under two sovereign masters; the pursuit of pleasure and the avoidance of pain” meaning that maximising pleasure is natural to human beings, which implies both that everybody knows the difference between right and wrong and that this could reasonably be described as “common sense.”  Yet this is not convincing because there is a gap between our instinct to pursue our own pleasure and knowing that we should maximise human pleasure in general.  Hume pointed out the is-ought gap in logic that affects Bentham’s reasoning and GE Moore dismissed classical Utilitarianism for relying on the Naturalistic Fallacy as well as for reducing goodness to what causes pleasure in a way that he felt to be far too simplistic.  Moore pointed out that it makes sense to ask if an instance of pleasure is good, as well as whether pleasure is good per se, which shows that pleasure is not in fact good in itself.  This means that many people might not know the difference between right and wrong by a utilitarian definition. Out instinct to maximise pleasure is not the same as a natural knowledge of the difference between right and wrong and the utilitarian moral imperative – to sacrifice our own interests when this would make more people happy – does not seem to be “common sense” to everybody! 

Further, drawing on Platonism GE Moore argued that all people recognise goodness when they see it, although goodness itself is indefinable. This is quite appealing because it explains why many people see fruitlessly altruistic actions – like a soldier getting blown up disobeying orders in trying to save an injured comrade – as good when this can’t be explained by other means. Yet, it is a stretch to say that everybody knows that such actions are good or that it is “common sense” when there is such a diversity of opinion and disagreement about this sort of issue.  While scholars like CS Lewis and Alastair MacIntyre were right to point out that some values and virtues are admired across ages and cultural boundaries, not all values and virtues are and certainly not by everybody.  If they were, there wouldn’t be any cultural disagreements over issues like polygamy and FGM and neither would there be any crime.  Wittgenstein’s approach of seeing meaning and truth depending on the rules of a “language game” and varying over time and between contexts makes best sense of the diversity that exists in the moral sphere, despite some basic agreement over the value of human life. This approach also casts doubt on both the claim that everybody knows the difference between right and wrong and the claim that such is “common sense”… because what is right and what is wrong depend on context and is neither objective, nor universal.

It seems that Hume’s sentimentalism or even AJ Ayer’s emotivism best explain why some people admire fruitless altruistic actions while others condemn them. By Hume’s analysis, our moral values represent our sentimental reactions to actions.  Surely, most people are repelled by murder, but this does not mean that there is something “wrong” about murder that can be “known,” only that many people share feelings about an action which often causes suffering. Further, by Ayer’s analysis, moral claims have no factual significance but are expressions of emotions and feelings in much the same way as hooray or boo might be.  Again, they don’t make ontological claims about actions, but only express approval or disapproval.  JL Mackie agreed with Ayer, arguing that moral values are “queer” in not belonging to any category of existence and that moral claims are based on errors.  For Hume, Ayer and Mackie, the difference between right and wrong is not something that can be “known” at all…. only felt.  The feeling that murder is wrong might appear to be based on “common sense” but this would apply only if “common sense” refers to popular opinion rather than the universal knowledge suggested by the wording of the title.  As RM Hare suggested, people might be prescribing actions when they say things like “murder is wrong”, rather than making a claim about murder itself, suggesting that a claim like murder is wrong has meaning while lacking factual significance. Yet, the fact that as CL Stevenson observed, moral claims are difficult to analyse and might in fact be doing a lot of different things suggests that it is wrong to say that everybody has knowledge of right and wrong, let alone that these are matters of “common sense”. 

In conclusion, while the claim that “we all know the difference between right and wrong; it is just common sense” is superficially attractive, it cannot withstand deeper critical analysis. For ethical realists, the claim goes well beyond what most would suggest.  For most ethical naturalists the knowledge of good and evil, while open to everybody, demands deeper analysis and study to attain.  Similarly, for most non-naturalists a priori knowledge of right and wrong goes beyond common sense and demands deeper thought.  While GE Moore did argue that it makes no sense to keep asking what goodness is because it is indefinable, an intuition… and we either recognise it or we don’t, even for him – the author of a famous essay called “Common Sense” (1905) knowledge of the difference between right and wrong is not the sort of gut reaction that this phrase might often suggest.  Also, for ethical non-realists the difference between right and wrong is neither something that has factual significance and can be “known” nor something that is agreed on in the way that “common sense” might suggest.