If one starts from a substantival stem, what must be the relation between the adjective and substantive derived from it? They must necessarily have the same sense, whichever of the two one considers the primary word; if _avara_ = _avaricious_, _avaro_ = _an avaricious person_; if _blinda_ = _blind_, _blindo_ = _a blind person_. This rule is all the more necessary in practice as there are a crowd of substantival stems concerning which one could not say whether they produce at first a substantive or an adjective: _vidva_ = _widowed_, _vidvo_ = _widower_; _nobela_ = _noble_, _nobelo_ = _nobleman_; _santa_ = _holy_, _santo_ = _a saint_. This is particularly true of the names of followers of this or that doctrine: _katoliko_, _katolika_; _skeptiko_, _skeptika_, etc. No one would think of using any suffix to derive one of these words from the other. There is only a very slight difference of meaning between a _katolika skeptiko_ and a _skeptika katoliko_, the substantive indicating in each case the primary and fundamental idea to which the other is superadded. This brings us to the enunciation of the _principle of reversibility_, which may be formulated as follows: "Every derivation must be _reversible_; that is to say, if one passes from one word to another of the same family in virtue of a certain rule, one must be able to pass inversely from the second to the first in virtue of a rule which is exactly the inverse of the preceding." That is an evident corollary of the _principle of uniqueness_, for otherwise one would be led to give two meanings to the same word. Let us suppose, for example, that from the noun _krono_, = _a crown_, one imagines it possible to derive directly (as is the case in certain languages) the verb _kronar_ = _to crown_. From this verb one could deduce inversely in virtue of the general rule the substantive _krono_ = _coronation_, so that the same word _krono_ would then mean both _crown_ and _coronation_. That would be, however, a logical error inadmissible in the international language, however numerous may be the examples of it which occur in living languages. On the contrary, thanks to the principle of reversibility, one can proceed from any word whatsoever of a family and arrive at any other word of the same family, or return to the initial word, in an absolutely unique manner, whereas if one did not observe this principle one would inevitably obtain two meanings for the same word.