Quintilian Institutio Oratoria 1.4.28

[28] while there are other verbs which resemble nouns, such as fraudator and nutritor. ¹ Again itur in antiquam silvam ² is a peculiar usage. For there is no subject to serve as a starting point: fletur is a similar example. The passive may be used in different ways as for instance in and in Yet a third usage is found in urbs habitatur, whence we get phrases such as campus curritur and mare navigatur. 29 Pransus and potus ³ have a meaning which does not correspond to their form. And what of those verbs which are only partially conjugated? Some (as for instance fero ) even suffer an entire change in the perfect. Others are used only in the third person, such as licet and piget, while some resemble nouns tending to acquire an adverbial meaning; for we say dictu and factu ⁴ as we say noctu and diu, since these words are participial though quite different from dicto and facto.