For from P7 it follows that a complex is self-identical iff the individual and haecceity which constitute it are such that the individual exemplifies the haecceity:
P7
wxyz
(w
.
X = y
.
Z
(w ex X & w ex Z & y ex X))
T18
xy
(x
.
Y = x
.
Y
x ex Y)
wxyz(w.X
= y.Z
(w
ex X & w ex Z & y ex X))