6.  But they contend that this title sets forth the nature of that to which it is applied; that the nature of the Spirit is not a nature shared in common with that of Father and of Son; and that, for this reason, the Spirit ought not to be allowed the common use of the name.  It is, therefore, for them to show by what means they have perceived this variation in the nature.  If it were indeed possible for the divine nature to be contemplated in itself; could what is proper to it and what is foreign to it be discovered by means of visible things; we should then certainly stand in no need of words or other tokens to lead us to the apprehension of the object of the enquiry.  But the divine nature is too exalted to be perceived as objects of enquiry are perceived, and about things which are beyond our knowledge we reason on probable evidence.  We are therefore of necessity guided in the investigation of the divine nature by its operations.  Suppose we observe the operations of the Father, of the Son, of the Holy Ghost, to be different from one another, we shall then conjecture, from the diversity of the operations that the operating natures are also different.  For it is impossible that things which are distinct, as regards their nature, should be associated as regards the form of their operations; fire does not freeze; ice does not warm; difference of natures implies difference of the operations proceeding from them.  Grant, then, that we perceive the operation of Father, Son and Holy Ghost to be one and the same, in no respect showing difference or variation; from this identity of operation we necessarily infer the unity of the nature.