Substance theory, or substance–attribute theory, is an ontological theory positing that objects are constituted each by a substance and properties borne by the substance but distinct from it. In this role, a substance can be referred to as a substratum or a thing-in-itself.

Substances are particulars that are ontologically independent: they are able to exist all by themselves. Another defining feature often attributed to substances is their ability to undergo changes. Changes involve something existing before, during and after the change. They can be described in terms of a persisting substance gaining or losing properties. Attributes or properties, on the other hand, are entities that can be exemplified by substances. Properties characterize their bearers; they express what their bearer is like.

Substance is a key concept in ontology, the latter in turn part of metaphysics, which may be classified into monist, dualist, or pluralist varieties according to how many substances or individuals are said to populate, furnish, or exist in the world. According to monistic views, there is only one substance. Stoicism and Spinoza, for example, hold monistic views, that pneuma or God, respectively, is the one substance in the world. These modes of thinking are sometimes associated with the idea of immanence. Dualism sees the world as being composed of two fundamental substances (for example, the Cartesian substance dualism of mind and matter). Pluralist philosophies include Plato's Theory of Forms and Aristotle's hylomorphic categories.

• EXISTENCE ( second order property )
  • General Existence
    • Concepts
    • Universals
  • Existence distinctions concerned with either
    • Abstract
    • concrete
  • Distinctions of existence whether
    • physical existence
    • mental existence
  • Distinctions of make up whether
    • materialistic , materialistic entity
    • immaterialistic , immaterial entity

• Variables
  • unknown variable
  • known variable
• parameter
  • known
  • unknown
• known
  • indefinite
  • definite
• unknown
  • indefinite , doubtful , suspicion
  • definite parameter

SINGULAR AND GENERAL EXISTENCE

Singular existence is the existence of individual entities while general existence refers to the existence of concepts or universals.

PHYSICAL AND MENTAL EXISTENCE

In contemporary philosophy, there are at least three prevailing ways to understand what a concept is:

ABSTRACT AND CONCRETE

Entities present in space and time have concrete existence in contrast to abstract entities, like numbers and sets. The type–token distinction identifies physical objects that are tokens of a particular type of thing. The type of which it is a part is in itself an abstract object. The abstract–concrete distinction is often introduced and initially understood in terms of paradigmatic examples of objects of each kind: