has the same identity. An object's identity may also be destroyed if the simplest description of the system at a point in time changes from identifying the object to not identifying it. Also an object's identity is created at the first point in time that the simplest model of the system consistent with perception identifies it. An object may be composed of components. A component is an object completely within the boundary of a containing object. Classical mechanics In classical mechanics a physical body is collection of matter having properties including mass, velocity, momentum and energy. The matter exists in a volume of three-dimensional space. This space is its extension. Interactions between objects are partly described by orientation and external shape. In continuum mechanics an object may be described as a collection of sub objects, down to an infinitesimal division, which interact with each other by forces that may be described internally by pressure and mechanical stress. Quantum mechanics In quantum mechanics an object is a particle or collection of particles. Until measured, a particle does not have a physical position. A particle is defined by a probability distribution of finding the particle at a particular position. There is a limit to the accuracy with which the position and velocity may be measured. A particle or collection of particles is described by a quantum state. These ideas vary from the common usage understanding of what an object is. String theory In particle physics, there is a debate as to whether some elementary particles are not bodies, but are points without extension in physical space within spacetime, or are always extended in at least one dimension of space as in string theory or M theory. In psychology In some branches of psychology, depending on school of thought, a physical object has physical properties, as compared to mental objects. In (reductionistic) behaviorism, objects and their properties are the (only) meaningful objects of study. While in the modern day behavioral psychotherapy it is still only the means for goal oriented behavior modifications, in Body Psychotherapy it is not a means only anymore, but its felt sense is a goal of its own. In cognitive psychology, physical bodies as they occur in biology are studied in order to understand the mind, which may not be a physical body, as in functionalist schools of thought. In philosophy A physical body is an enduring object that