  As part of a probabilistic reconstruction of quantum theory (QT), we show
that spin is not a purely quantum mechanical phenomenon, as has long been
assumed. Rather, this phenomenon occurs before the transition to QT takes
place, namely in the area of the quasi-classical (here better quasi-quantum)
theory. This borderland between classical physics and QT can be reached within
the framework of our reconstruction by the replacement $p \rightarrow M (q,
t)$, where $p$ is the momentum variable of the particle and $M(q, t)$ is the
momentum field in configuration space. The occurrence of spin, and its special
value $1/2$ , is a consequence of the fact that $M(q,t)$ must have exactly
three independent components $M_{k}(q,t)$ for a single particle because of the
three-dimensionality of space. In the Schr\"odinger equation for a "particle
with spin zero", the momentum field is usually represented as a gradient of a
single function $S$. This implies dependencies between the components
$M_{k}(q,t)$ for which no explanation exists. In reality, $M(q,t)$ needs to be
represented by three functions, two of which are rotational degrees of freedom.
The latter are responsible for the existence of spin. All massive structureless
particles in nature must therefore be spin-one-half particles, simply because
they have to be described by $4$ real fields, one of which has the physical
meaning of a probability density, while the other three are required to
represent the momentum field in three-dimensional space. We derive the
Pauli-Schr\"odinger equation, the correct value $g=2$ of the gyromagnetic
ratio, the classical limit of the Pauli-Schr\"odinger equation, and clarify
some other open questions in the borderland between classical physics and QT.
