The Golden Rule
The golden rule has been a foundational principle for moral behavior for as long as humans have recorded their ethical guidelines, and likely even longer. It is the single rule needed to determine right behavior:
"Act only according to that maxim whereby you can at the same time will that it should become a universal law."
Examples:
Racism and Slavery: A racist might say, "We should make it a universal law to enslave everyone who doesn't have the same skin color as me." Historically, if the majority in a society agrees with this sentiment, it leads to such practices. If you consider this wrong, you must counter with, "We should make it a universal law to stop anyone who advocates for such actions."
Theft: One might argue, "We should make it a universal law that it is right for me to steal in this specific situation." Specific applications, as well as broad ones, fall under the golden rule. While a general maxim like "You shouldn't steal" holds, exceptions can be morally justified. For instance, if you are starving and steal a loaf of bread from a baker, it may not be wrong under these dire circumstances.
In essence, the golden rule guides us to consider whether the principles behind our actions could be consistently applied as a universal law, helping us navigate both broad ethical principles and specific situational ethics.