The auxiliary verb （助動詞）～（さ）す is added to the mizenkei of verbs.  ～す is added to yodan, na-column irregular, and ra-column irregular verbs while ～さす is added to other conjugations, namely upper and lower monograde and bigrade, na-column irregular, and sa-column irregular verbs (however, the せ that is expected in せさす and other sa-column verbs is almost always dropped, leaving さす,  旅さす, and the like).  It’s conjugation is that of a lower-bigrade verb.  Because ～さす and ～す conjugate the same, I’ve treated them as one single auxiliary verb.

It has two uses:

• 使役  Causative and Permissive Voice

Much like its modern analog, the main function of ～（さ）す is to convey the causative and permissive moods, which are, of course, a single combined mood in Japanese.  It’s modern translation is <未然形>（さ）せる and its English translation is “to let <verb>; to make <verb>.”

飲ます （飲ませる）(I make/let him drink.)

食べさせたり （食べさせた）(I made/let him eat.)

見さすれども（見させたけれども）(I made /let him look, but…)

• 二重尊敬 (Hyper-honorific)

This use shows the greatest respect and is therefore only used when the subject of the sentence is a person of very high rank, such as the emperor and empress, prince and princess, and some other high ranking noble positions.  ～（さ）す cannot indicate the honorific alone; it serves only to further raise the respect. Therefore, this use accompanies other honorific verbs and in the case of their absence, the meaning defaults to the 使役. In its use with 給ふ・たまふ, it’s meaning is almost certainly this honorific.

Ex) 矢を射させた給ふ （矢をお射になる）(He shoots an arrow (hyper hon).)

Due to modern Japanese lacking an honorific form of this level, 「お<連用形>になる」 is the closest we can get.

Warning

Do not confuse transitive verbs ending in ～す with this construction. Their conjugation is different and the regular verb naturally does not carry a causative meaning.

Ex) trans  出さむ、出したり、出す、出すは、出せども、出せ

causative   出させむ、出させたり、出さす、出さするは、出さすれども、出させよ

The target of causative verbs-

The target of causative verbs, ie. the receiver of the verb, is marked using several different means in classical Japanese. First, let’s make sure we can identify the target.

modern 私 は子供（を・に）食べさせた。BUT 私 は子供に野菜 を食べさせた。

(Disclaimer: the following terminology refers only to the Japanese construction. In English, we form the causative differently so these words get switched around). In the above sentence, the italicized noun is the subject/causer/agent, the bold word is the causee/patient/target, and the bold italicized word is the object. Note: this changes in causative-passive construction, so don’t get that confused! Therefore, the causer  is marked by は・が, the causee by に・を, and the object by を.

The marking of the causee in the classical causative phrase sometimes depends on a verb’s transitivity. Instead of going through each, I’ll just list the possible case markers according to verb type.

trans に、をして、して
intrans に、を、をして、して

To put it simply, the causee of intransitive verbs is marked by either に or を and obligatorily に if the object is explicit (like in modern Japanese).  The causee of transative is invariably marked by に. The particles をして and the shortened して mark the causee for both, but never mark the object.