Complex problems are questions or issues that cannot be answered through simple logical procedures. They generally require abstract reasoning to be applied through multiple frames of reference.
Complex problems can be of an epistemological nature—involving questions related to the nature and scope of human knowledge—or they can involve philosophical questions concerning ontology or the nature of being. Epistemologically complex problems may concern, for example, the status of an object of art. What constitutes a work of art is a problem that cannot be sufficiently answered through axiomatic or reductive thinking. Only answers that addresses art's role within society, its philosophical consequences and its historical development—in short, a multifarious and highly complex set of concerns—are adequate. Methods of solving complex epistemological questions such as these include systems theory, an approach developed in the mid-20th century that attempted to provide interdisciplinary solutions to complex problems.
In terms of ontological problems, the understanding of the nature of being is a complex problem that remains unresolved in philosophy. For 18th century German philosopher Immanuel Kant, transcendental idealism was the solution to the ontological problem of being. In his understanding, human subjects could only apprehend phenomena, or sensory impressions of the world; the true kernel of being, which exists in the form of noumena, or "things-in-themselves," is inaccessible to human perception or cognition.