The principle of reversibility fixes the rules of direct derivation for the cases which are the converse of those we have studied. Just as the substantive directly derived from a verb denotes the state or action expressed by this verb (or, more strictly, by its root), so a verb can be derived directly from a substantive only if the latter expresses an action or a state. For example, _paco_ = _peace_; can one form the verb _pacar_, and if so what will be its meaning? This verb can only signify one thing, _to be in the state of peace_, and not _to pacify_ or _make peace_, for in that case _paco_ would mean _pacification_ or _conclusion of peace_, and not the _state of peace_. Similarly, if one can and must convert an adjective into a noun by the simple substitution of _-o_ for _-a_, the adjective immediately derived from a substantive can only mean "what is --." If _homo_ = _a man_ (a human being), _homa_ can only mean _human_ in the sense of _which is a man_ (human being); _homa ento_ = _a human being_. But if one wishes to obtain an adjective signifying "which belongs to --," "which relates to --," "which depends on --," it is necessary to employ a suffix (_-al_): _homala manuo_ = _a human hand_. One might equally well say _manuo di homo_ = _the hand of a man_ (human being). But just as the preposition _di_ is indispensable for indicating the relationship between two ideas which are not simply juxtaposed, but depend on each other, so, if we wish to express one of the ideas in adjectival form, we require a suffix which also expresses this relation or dependence. Besides, a suffix of this nature exists under different forms in all our languages: G. _-isch_; E. _-ic_, _-al_, _-ical_; F. _-ique_, _-al_, _-el_; I. _-ico_; S. _-ico_. The choice of _-al_ rather than _-ik_ was determined by reasons of euphony and also internationality, the derivative adjectives employed in science (the most international of all) ending often in _-al_: _mental_, _vocal_, _spatial_; _rationnel_, _universel_, _fonctionnel_, etc.