Metaphysics is the study of the most general features of reality, such as existence, time, the relationship between mind and body, objects and their properties, wholes and their parts, events, processes, and causation. Traditional branches include cosmology and ontology.
Idealism is the belief that reality is fundamentally mentally constructed or otherwise immaterial while realism hold that reality, or at least some of it, exists independent of the mind. Subjective idealism describes objects as no more than collections or “bundles” of sense data in the perceiver. George Berkeley expressed how existence is tied to perception with Esse est aut percipi aut percipere or “To be is to be perceived or to perceive”. Particulars are those objects that are said to exist in space and time, as opposed to abstractions, such as numbers. Universals 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. Realism is sometimes used to support their existence while nominalism is sometimes used to stand for the denial of universals, abstractions or both. Conceptualism holds that universals exist, but only within the mind’s perception.
The question of whether or not existence is a predicate has been discussed since the Early Modern period. Essence is the set of attributes that make an object what it fundamentally is and without which it loses its identity. Essence is contrasted with accident: a property that the substance has contingently, without which the substance can still retain its identity.