These groupings allow philosophers to focus on a set of similar topics and interact with other thinkers who are interested in the same questions. The groupings also make philosophy easier for students to approach. Students can learn the basic principles involved in one aspect of the field without being overwhelmed with the entire set of philosophical theories. Various sources present different categorical schemes. The categories adopted in this article aim for breadth and simplicity. These five major branches can be separated into sub-branches and each sub-branch contains many specific fields of study:  These divisions are neither exhaustive, nor mutually exclusive. (A philosopher might specialize in Kantian epistemology, or Platonic aesthetics, or modern political philosophy). Furthermore, these philosophical inquiries sometimes overlap with each other and with other inquiries such as science, religion or mathematics. Metaphysics is the study of the most general features of reality, such as existence, time, objects and their properties, wholes and their parts, events, processes and causation and the relationship between mind and body. Metaphysics includes cosmology, the study of the world in its entirety and ontology, the study of being. A major point of debate is between realism, which holds that there are entities that exist independently of their mental perception and idealism, which holds that reality is mentally constructed or otherwise immaterial. Metaphysics deals with the topic of identity. Essence is the set of attributes that make an object what it fundamentally is and without which it loses its identity while accident is a property that the object has, without which the object can still retain its identity. Particulars are objects that are said to exist in space and time, as opposed to abstract objects, such as numbers, and universals, which are properties held by multiple particulars, such as redness or a gender. The type of existence, if any, of universals and abstract objects is an issue of debate. Epistemology is the study of knowledge (). Epistemologists study the putative sources of knowledge, including intuition, a priori reason, memory, perceptual knowledge, self-knowledge and testimony. They also ask: What is truth? Is knowledge justified true belief? Are any beliefs justified? Putative knowledge includes propositional knowledge (knowledge that something is the case), know-how (knowledge of how to do something) and acquaintance (familiarity with someone or something). Epistemologists examine these and ask whether knowledge is really possible. Skepticism is the position which doubts claims to knowledge. The regress argument, a fundamental problem in epistemology, occurs when, in order to completely prove any statement, its justification itself needs to be supported by another justification.