Syriac fathers borrowed philosophical terminology from the Greeks, especially Aristotle. They also had their native sense about philosophy going back centuries to the ancient thinkers of their land. Therefore, it is important to study how the early Syriac Fathers used the terms the Church uses to describe nature and person as in “two natures in one person in Christ”.
Saint Ephraim the Great in On the Hypostasis of the Holy Trinity, writes regarding the trinity, but defining the same words later used to speak of Christ (my translation):
You have heard of the Father, Son and Holy Spirit. These three gain their respective names due to their qnume (hypostasis). It is not that the names are mingled and shared appellations of one entity, but that the three—Father, Son and Holy Spirit—are mingled in unity [they can be spoken of as one]. If you confess that these three names are true, but you do not confess their qnume, then you are just confessing an empty name and are an unbeliever. Anything which you name which does not have a qnuma is just a meaningless name, empty of reality. Anything that does not have a qnuma is just an empty name. A qnuma is what is required to minimally assert that something exists in reality.. (ܡܪܓܢܝܬܼܐ ܕܥܠ ܫܪܪܐ ܕܟܪܣܛܝܢܘܬܐ.ܩܫ ܝܘܣܦ ܒܝܬ ܩܠܝܛܐ ܡܘܨܠ ܡܛܒܥܬܐ ܐܬܘܪܝܬܐ1924 pg 114)
For Saint Ephraim, qnuma, which we call hypostasis, is the reality of a nature. However If one speaks in abstract of human nature, kyana is appropriate. Kyana describes humanity as a whole. But if I speak of something as being human, it must have a qnuma, or it does not exist. Here is another quote from Saint Ephraim: “For where there is no qnuma, namely, hypostasis, (or the underlying substance of nature) its appellation is null and void, for that which has no qnuma, its name is also void.” (The Book of Marganita. Mar Eshai Shimun XXIII, Kerela 1965, pg 81)
Almost a thousand years later, Mar Yokhannan Bar Zo’by wrote a treatise on “Nature as separate from Hypostasis and Hypostasis from Prosopon and Prosopon from Appearance”. The English translation of the Syriac title that this treatise appears in is The Book of Harmonious Tapestry (translation mine).
For kyana (nature) is different in its essence from qnuma (hypostasis). Kyana is universal, but qnuma is specific. When kyana is divided, it constitutes its various [instances] and qnume. But when qnuma is divided, it is corrupted and destroyed because if you divide qnume…it will lose its natural qualities. (ܡܪܓܢܝܬܐ 98)
When Chalcedonian Orthodox use the corresponding term, Hypostasis, we mean a person. Such as the person of Jesus Christ, in whom we speak of two separate natures—divine and human. Nature to use is one word used to speak of the universal as well as the instance of a nature.