  In this paper, without any priori assumption about the post-measurement state
of system, we will examine how this state is restricted by assuming each of
these following assumptions. First, by using this reasonable assumption that
two successive measurements should be describable as one measurement. Second,
by assuming the impossibility of faster than light signaling, "No-signaling
condition". However, only by using these assumptions it is not possible to
obtain the usual projection postulate. Instead, by means of a simple lemma, we
will show that the density operator of system after a measurement is a linear
function of the density operator determined by the usual post-measurement
postulate. Furthermore we will show this linear function has a Kraus
representation. Finally, we will discuss about the physical meaning of this
consequence.
