Identifying the "greatest" problem in philosophy may be subjective, but these three long-standing problems have long attracted philosophical discourse:
Methodology:
What is the essence of reality? This question explores the fundamental nature of existence, including the essence of being, the existence of a higher force or ultimate reality, and the relationship between mind and matter.
Theory of consciousness:
What is knowledge and how to acquire it? This field studies the nature, scope and limitations of knowledge, including questions about perception, reason, truth, belief and skepticism.
Ethics:
What is the essence of right and wrong? Ethics encompasses the study of morality, dealing with the ethical framework, the nature of good and evil, the foundations of ethical judgment, and how individuals should act in various circumstances.
These questions have been explored by philosophers across centuries and cultures, and the various perspectives and methods have formed a continuous dialogue in an attempt to unravel these complex and fundamental mysteries of human existence.