Humans have long pondered not only how we came to be but also why we came to be. The earliest Greek philosophers focused their attention upon the origin and nature of the physical world; later philosophers have theorized about the nature of knowledge, truth, good and evil, love, friendship, and much more. Philosophy involves a methodical assessment of any and all aspects of human existence and experience. The realms of philosophy and religion have sometimes intersected in conducting such inquiries as these. As with philosophy, the study of religion underscores how humankind has long speculated about its origins. The possibility of a higher being (or beings) to which livings things owe their existence has long captived human thought. Many religions also offer their own views on the nature of good and evil, and they may prescribe guidelines and judgment on different kinds of human behavior.