Utilitarianism suggests that we should do what is best for society, or the greatest number of people (Heavey and Gianoutsos, 2013, p. 242-243). (In the haircut example, perhaps that would mean telling your friend to fix their hair, so no one else has to suffer in looking at it!) This principle makes logical sense, but can be difficult to put into practice when the dilemma involves one person who is close to us vs. the unknown and unnamed mass of society.

Hedonism suggests that "pleasures are intrinsically good" and therefore we should do what gives us pleasure (Heavey and Gianoutsos, 2013, p. 243). Of course, that doesn't help us when we have decisions to make that offer little pleasure (like the infamous trolley problem), and it cuts us off from a range of unpleasurable emotions (frustration, confusion, anger, or fear) that can actually help us to grow and develop.

The categorical imperative, developed by Immanuel Kant, suggests that we should act in such a way that we are comfortable with that act becoming a universal law. What this means is that the motivations or consequences of the act don't matter as much as the act itself. If an act (like lying) is wrong, it is wrong in all situations, from lying about a friends' haircut to lying for financial gain.

Relativism is "the view that there are no moral values or standards applicable to all peoples, cultures, and societies" (Heavey and Gianoutsos, 2013, p.242). In other words, what is ethical and good in one culture could become wrong in the context of another culture. Globalization and international connections can make this principle challenging, especially in the case of human rights violations.

Natural law is a concept that essentially says, if it happens a certain way in nature, then that is good and right. Diverging from natural law creates negative consequences. Critics point out that this perspective might hinder scientific and technological (i.e., non-natural) research; additionally, what is "natural" is often debated (Heavey and Gianoutsos, 2013, p.243-244).

Existentialism is the belief that there are no external forms (i.e., God, nature, culture) to provide meaning; instead, "it is up to each individual" to create meaning for themselves (Heavey and Gianoutsos, 2013, p.244). Existentialism "demands complete personal responsibility for one's actions and their consequences" (Heavey and Gianoutsos, 2013, p.244).