Varro, de Lingua Latina 6.7:
“If I, knowing a thing, say it (dico) to someone who does not know it and impart to them what they were previously ignorant of, you may see the derivation of the verb ‘teach’ (doceo). This is either because we speak (dicimus) when we teach (docemus), or because those who are taught (docentur) are being led into (inducuntur) that which they are being taught. From the fact that one knows how to lead, one is a leader (dux aut ductor); and the word doctor is one who leads students in such a way that he teaches (doceat) them. From leading (ducendo) comes teaching (docere) and from discipline (disciplina) comes learning (discere), with a few letters changed. From the same principle we have documents (documenta), which are examples spoken for the sake of teaching (docendi).”
Si dico quid sciens nescienti, quod ei quod ignoravit trado, hinc doceo declinatum vel quod cum docemus dicimus vel quod qui docentur inducuntur in id quod docentur. Ab eo quod scit ducere qui est dux aut ductor; hinc doctor qui ita inducit, ut doceat. Ab ducendo docere disciplina discere litteris commutatis paucis. Ab eodem principio documenta, quae exempla docendi causa dicuntur.