The specific issues of philosophy are constantly evolving. As human knowledge progresses in fields such as biology and psychology, new questions and issues in philosophy are raised. Virtually all new issues in philosophy, however, can be seen as modified forms of old philosophical issues. Most philosophical issues today can be categorized within the main branches of philosophy.
Epistemology is the branch of philosophy concerned with the origin and nature of knowledge itself. The main issues of epistemology concern the meaning and scope of human knowledge, as well as the ways in which people acquire knowledge. One of the main schools of thought in epistemology is empiricism, which asserts that knowledge is gathered through sensory experience. In contrast, rationalism asserts that knowledge is derived from mental reasoning.
Ethics is the branch of philosophy that is concerned with questions of morality. This includes concepts such as good versus evil and right versus wrong. Theories of ethics look to find a base for classifying human behavior within these concepts. Aristotle wrote one of the most famous theories of ethics. Aristotelian ethics is based on the idea that the ultimate "good" is happiness and fulfillment through the development of a person's true nature and identity. A contrasting example is hedonism, which is an ethical theory based on the maximization of pleasure and minimization of pain.
Metaphysics focuses on the true nature of existence. This includes theories concerned with the physical reality of space and time, as well as theological theories that may posit a reality beyond the physical one people experience. One of the most famous theories in metaphysics was written by Plato, who asserted that the reality humans perceive is analogous to a shadow or reflection of the true, essential reality of the universe.
Aesthetics analyzes the nature of beauty, such as in the appreciation of art. Theories in aesthetics analyze the ways in which people appreciate beauty, as well as their justifications for deciding what is and is not beautiful. Traditionally, theories of aesthetics had an abstract nature. Some modern theories of aesthetics, however, incorporate neurological knowledge in an aim to scientifically explain the human mind's appreciation of art and beauty.