It must be confessed that it is not easy to understand how this or any other _substantial_ mode can be _really_ distinct from the substance it modifies. And in truth the distinction is not real in the full sense: it is not between _thing_ and _thing_, _inter rem et rem_. All that is claimed for it is that it is not merely mental; that it is not merely an _ens rationis_ which the mind projects into the reality; that it is a positive perfection of the nature or substance, a perfection which, though naturally inseparable from the latter, is not absolutely inseparable, and which, therefore, is _de facto_ supernaturally absent from the human nature and replaced by the Divine Personality in the case of the hypostatic union. It belongs, moreover, to the order of substance, not to that of accidents: the substantial mode differs from the accidental mode, or modal accident, in this, that it gives to the substance some ultimate determining perfection which appertains to the substance as such, and whereby the substance is completed in the order of “existing in itself”. Subsistence is not an accident, even though it supervenes on the complete nature, for it determines the substance of the latter, not in relation to any line of accidental activity, as a power or faculty, nor as something modifying it accidentally, but as a mode which ultimately determines and perfects it in the order of substantial reality itself, in the order of “existing in itself” in such a full and perfect manner as to be _sui juris_ and incommunicable. The main difficulty against this view is also theological: If subsistence is a positive perfection it either belongs to the complete individual nature or it does not; in the former case the humanity of Christ, assumed by the Divine Word, was not a complete human nature; in the latter case the individual human nature can exist without it: and both consequences are equally inadmissable. But it may be replied that, granting the first member of the disjunctive, the consequence inferred from it does not really follow: subsistence belongs to the complete individual nature as an ultimate natural complement; but when it is absent and supplied supernaturally by the Divine Personality the nature is still complete as a nature: it is wanting in no absolute or entitative perfection, but only in a modality which is supereminently supplied by the Divine Personality. Neither is the consequence from the second member of the disjunctive a valid inference. For though personality as a mode does not belong to the essence of an individual human nature, no such individual nature can exist without _some_ personality, either its own or another: just as extension cannot exist without _some_ shape, though any particular shape is not essential to it.