Some philosophers believe morality can be determined by observing the world around you. What is this philosophy called?

Answer: Moral relativism

Moral (or ethical) relativism asserts that that there are no "eternal" truths in regard to morality - - things that can be applied to all people of all times in all cultures or contexts. Instead, morality is understood in terms of what is seen as acceptable or noble in the world in which you are living. In this approach, cultural norms have much to do with establishing what is considered "moral" in any particular society. This view of morality thus accounts for why and how beliefs about right and wrong can be seen to shift and change over time within societies.

Ethical thinkers who object to moral relativism would argue that the relativists' approach ultimately means there is no true "morality," because then there is no standard by which right and wrong can be assessed on a lasting basis.