The Compendium of Caraka 6.3.33

6.3.33

antarvego bahirvego dvividhaḥ punarucyate/
prākṛto vaikṛtaścaiva sādhyaścāsādhya eva ca//

it is again spoken of as two kinds: of internal speed or external speed; / natural or unnatural; curable or incurable;