  We show that it is possible to explain the quantum measurement process within
the framework of quantum mechanics without any additional postulates. The key
concept of the theory is decoherence, which appears as an inherent
characteristic of quantum mechanics and results from the uncertainty relation.
In contrast to environment-induced decoherence, this decoherence exists prior
to a measurement being made. To clarify our idea, we examine three elemental
experiments: a Stern-Gerlach-like experiment, the Einstein-Podolsky-Rosen-Bohm
(EPR-Bohm) experiment, and the double-slit experiment. By considering the first
experiment, we explain how the uncertainty relation between position and
momentum introduces decoherence prior to measurement. Consideration of the
EPR-Bohm experiment leads us to conclude that the correlation of the EPR pair
is not a consequence of what is known as the collapse of the wave function. Our
theory of decoherence can also be applied to experiments with continuous
eigenvalues, such as the double-slit experiment. Consideration of the
double-slit experiment leads us to understand how pure quantum mechanics
describes the fact that quanta behave as interfering particles.
