Siger of Brabant,
Quaestiones in tertium de anima, Louvain 1972, 79-80, ll.43-66.

“Praeterea, alia est ratio essendi formae materialis et compositi seu
formae per se subsistentis. Ratio enim essendi formae materialis est secundum
quam est aliquid aliud, ut ratio compositionis est secundum quam habet esse
compositum, et ratio figurae secundum quam habet esse figuratum; unde ratio
essendi formae materialis est quod sit unita alii. Ratio autem essendi
compositi vel formae liberatae a materia est quod sit ens per se et separate,
non unum ens cum alio. Ex hoc sic arguitur. Cum cessat ratio essendi alicuius,
ipsum corrumpitur et non est; sed cum separatur forma materialis a materia,
cessat eius ratio essendi, ut ex praedictis apparet; nulla igitur forma cuius
separatio a materia non est sua corruptio est materialis. Sed separatio animae
intellectivae a corpore et materia non est eius corruptio. Ergo non habet esse
unitum ad materiam. Et declaratur ratio in exemplo. Cum ligna, lapides et
lateres in domo cessant esse composita a forma compositionis eorum, cessat esse
compositio; et cum secundum figuram cessat aliquid figuratum esse, cessat
esse figura. Et similiter est de forma substantiali ad illud cuius est forma,
quod, cum secundum eam cessat esse materia, cessat esse formae materialis,
licet notius sit in accidentali forma quam substantiali. Et sunt istae rationes
essendi, qua aliquid habet esse unite ad materiam et qua aliquid habet rationem
subsistentis per se et separate, oppositae adeo quod eidem inesse non possunt.
Unde anima intellectiva non potest habere rationem per se subsistentis et, cum
hoc, unum facere cum materia et corpore in essendo.”

The ratio essendi [raison d’être, mode of being] of a material form is
different from the ratio essendi of
a composite substance or of a subsistent form. For the ratio essendi of a material form is that of
something on account of which [secundum quam] something else is, just as the ratio of
composition is that it is something on account of which a composite thing is, and the ratio of shape is
that it is something on account of which a thing is thus and so shaped;
therefore, the ratio essendi of
a material form is that it is united with something else. But the ratio essendi of a composite substance or of a
form that is free from matter is that it is a being per se and
separately, not one being with something else. From this we argue as follows.
When the ratio essendi [raison
d’être] of something ceases to be, then it is destroyed and is not; but when a material form is separated
from matter, then its ratio essendi ceases to be, as is clear from
what has been said; therefore, no form whose separation from matter is not its
corruption is a material form. But the separation of the intellective soul from
the body and matter is not its corruption. Therefore, it does not have being
united with matter. And the reasoning is clarified by means
of examples. When the timbers, stones, and bricks in a house cease to be
composed on account of their form of composition, the composition ceases to be;
and when something ceases to be thus and so shaped on account of its shape,
that shape ceases to be. And the case is similar with the substantial form,
with respect to that of which it is the form; for when matter ceases to be actual on account of this form, then this material form
ceases to be, although this is better known in connection with an accidental
form than in connection with the substantial form. And these rationes essendi, on account of which something
is united with matter, and on account of which it is a per se and
separately subsisting being, are opposite to such an extent that they cannot
belong to one and the same thing. Therefore, the intellective soul cannot have
the ratio of a per se subsisting being and, at the same time,
constitute with matter and the body a thing that is one in being.