2160. The infinitive is in its origin a verbal substantive.
2161. The present infinitive active is an ancient dative, closely
resembling in meaning and use the English infinitive with _to_. It
originally marked action merely in a general way, without indication of
voice or tense. In virtue of this original timeless character, the
present often represents action which is really past or future; in such
cases the time must be inferred from the context.
2162. The present infinitive active gradually approached the character
of a verb, and the original substantive nature being forgotten, it was
supplemented by a passive, and by forms for completed and for future
action, active and passive.
2163. The infinitive has furthermore two other properties of the verb:
(_a._) it is modified by an adverb, not by an adjective; and (_b._) it
is followed by the construction of its verb.