[Excluded by Monophysite teaching] was any notion that the union was not merely a union of two natures but of two ousias, divine and human; for if "ousia" were taken in its usual and proper significance as referring to that which was common to all members of a class, a union of ousias would mean that "the Holy Trinity has become incarnate in all of humanity and has become composite with our entire race," which would be blasphemous (p. 58).But this could only be objectionable, if I understand things rightly, if we have first presumed that the "ousia" of humanity is a thing having real existence - following Plato.
In this way I think we can see that philosophy informs theology. Because of certain philosophic preconceptions they held, the Monophysites stumbled over the two natures of Christ. Something similar could be said for the Nestorians; they held (cf. p. 39ff.) that a hypostasis is part of the nature, so that it would be impossible for Christ to possess a single hypostasis and at the same time to possess two natures; Pelikan quotes them as saying that it was "'an insane error' and 'a corruption of our faith'" to suppose otherwise.
This is why it's important to hold to the dogmas of the Faith above all else, and it's why reason must submit to the Faith too. Christ has two natures, but he is one person; hence the Nestorians should have accepted that their notion of human nature was erroneous on the question of the hypostasis. Christ has two natures, but he is not incarnate in all humanity; hence the Monophysites should have modified their view of what a union of the two natures implied.
In a day or two I'll have another example of how dogmas necessarily inform our conclusions about other things.