Now it follows from this principle that it is quite incorrect to say, as is often done, "Being given a stem, it suffices to add to it _-ar_ to form a verb, _-o_ to form a substantive, _-a_ to form an adjective"; we require to define the sense possessed by this verb, substantive, and adjective. In other words, to every derivative of form there must correspond a derivative of sense which is in no wise arbitrary, but determined by general rules. If _dorm-ar_ = to sleep, _dorm-o_ cannot mean indifferently the sleeper, the dormitory, or the desire to sleep; if _blind-a_ = blind, _blind-o_ cannot signify at pleasure either blindness or the act of blinding. The rule which must guide us here is the principle enunciated above, namely, that a stem always preserves the same sense and expresses the same idea; if one wishes to express another idea related to the former in a definite way, it is necessary to add to the stem a morpheme expressing this relationship. The morphemes which denote the relations of our ideas are the affixes of derivation, which permit us to express a whole family of ideas by the aid and as the function of one fundamental idea, and to form correspondingly a family of words all derived from the same stem, as occurs, as a matter of fact, in natural languages. Certain of these affixes are wrongly classed amongst the grammatical inflections, such as, for example, the participial suffixes which serve to derive an adjective or a substantive from a verb, denoting him who performs the action, or is affected by (subject to) the state or relationship expressed by the stem: _dorm-ant-a_ = _sleeping, arol-ant-a_ = _speaking_, whence, by simple change of the final letter, _dorm-ant-o_ = _sleeper_, _parol-ant-o_ = _speaker_. One will perceive thereby the difference between _direct_ derivation, which is effected by means of the grammatical inflections, and _indirect_ derivation, which is effected by means of the addition of affixes. There is nothing arbitrary about this distinction, for it rests on the logical principles enunciated above, which determine the theoretical and practical value of the international language. From these principles follow at once the rules of direct derivation. If one starts from a verbal stem, what must be the sense of the substantive directly derived from it? This sense can be none other than the state or action expressed by the verb: _dormar_ = _to sleep_, _dormo_ = _sleep_; _parolar_ = _to speak_, _parolo_ = _a word_; _frapar_ = _to strike_, _frapo_ = _a blow_. In these derived words we perceive the sense of the verb stem, and the proof of that is that in our natural languages we often employ the infinitive for this purpose: _le manger_, _le boire_, _le dormir_, _le rire_; _das rennen_ (in English the verbal in _-ing_ is employed with the sense of the infinitive). Indeed, one might completely identify the verbal substantive with the infinitive.