For guidance, then, a biologist turns to moral philosophers. Yet even after centuries of reflection and debate, philosophers themselves do not agree on core ethical principles for defining "good." They generally recognize, however, three basic approaches. One approach, consequentialism, focuses on the outcomes themselves. For example, morality is assessed as the greatest good for the greatest number. Good may be defined variously as benefit, happiness or pleasure. A contrasting approach, deontology, emphasizes instead motives (or reasoning). For example, morality is measured by feelings of sympathy or virtuous intent. Both approaches draw on widely shared intuitions, although they sometimes lead to different moral codes. They have not yet been synthesized. A third, complementary strand of philosophical thought situates morality on yet another level: the concept of a social contract. Morality is characterized primarily by mutual consensus on values. Biologists can inform each perspective, as described in what follows in separate sections (Figure 1).