Ethical principles are maxims for action. They are part theoretical in that they derive from an ethical system, but they are also part practical, because they are capable of being put into action. An ethical system is worthless unless it provides basic foundational ideas that can be applied in many difficult cases.
Immanuel Kant made universality the central maxim of all moral judgment. The basic idea is that an act is good when it can, without absurdity, be turned into a universal law. A universal law is one that can be binding upon anyone. If you seek to cheat someone out of money, you ask yourself whether this can be a universal rule. It cannot, since if everyone cheated in this way, the economy would collapse. No one would trust each other. It is inherently evil for this reason and, hence, immoral. If an action does not pass the universality test, it is immoral.
Many more radical theories have placed a strong emphasis upon labor as an ethical principle. Labor becomes ethical when it becomes a part of how humanity is creating and defining itself. Rather than being pure drudgery, work becomes something positive, a way to reshape nature to conform to basic human needs. John Locke, for example, famously held that when you put your labor into nature, what you have created becomes your property. Your property is justified because you have made it; you have created it. Work here is a means to expand your mind, to create property and wealth and to make nature work for man, rather than against him.
Reason is the principle of thought. It, as taught by writers such as Plato and St. Augustine, serves to put a brake on passions such as anger, greed and lust. Reason is the principle of control — it places the passions in their proper position and keeps them from taking over the entire soul. It demands moderation, as Aristotle famously taught. For example, Aristotle writes that courage is a mean, a mean between foolhardiness on one extreme and cowardice on the other. Many virtues can be considered a mean between two extremes. These extremes are dominated by passion such as anger and fear.
Integrity derives from the verb “to integrate.” It is a central ethical principle because it suggests that the personality is genuine and true. The integral personality is one that is based around several ideas, a mission, a strong sense of self that is present at all times. Its opposite is one that wears “masks,” that tells people what they want to hear and disguises its intentions and opinions. The opposite of the integral personality is the dissimulated personality. This is basic social honesty, where you believe in your purpose and moral ideas and do not try to disguise them. The dissimulating personality is dishonest in that it mirrors its surroundings, conforming to what is popular for the sake of social acceptance.