פירוש על ויקרא 21:9

כי תחל לזנות means, if she profanes herself through unchastity, i. e. that a marriage-tie of some kind applied to her and she nevertheless prostituted herself either after betrothal (which constituted a kind of marriage-tie, since she would require a bill of divorce to enable her to marry someone else) or after marriage (having left her father's house for her husband’s). Our Rabbis are of different opinions regarding this (whether Scripture speaks here of an ארוסה or a נשואה), but all agree that Scripture is not speaking of a פנויה (a woman who is neither betrothed nor married) (cf. Sanhedrin 51b).

ובת איש כהן, a girl who is already betrothed to be married. The details of this legislation are explained in Sanhedrin 50.

באש תשרף, “she is to be consumed by fire.” Had the Torah written the sequence תשרף באש, I would have understood that the meaning is that the daughter mentioned is to be literally burned to death on a pile of wood set afire. The Torah’s method of execution by what is called שרפה is not similar to burning someone on a pyre. The body of the guilty party is most certainly not destroyed by fire. I have found that the sages in Pessachim 75 when discussing how the Passover is to be prepared for eating, discuss the finer points of what is written. The Torah had stipulated that the Passover has to be eaten after it has been צלי אש, “roasted over the fire.” The sages there say that the words באש תשרף “she is to be burned by fire,” include all possible methods of burning as long as the course of the burning is fire. Rav Matnah stated that the way this penalty is applied is through administering molten lead to the party to be executed and pouring it into the larynx so that instant death occurs. He points to the difference in the wording of the Torah in Leviticus 4,12 when the parts of a bull which serves as a sin-offering for the High Priest is burnt outside the camp except for the parts offered on the Altar. The wording there is that ושרף אותו על עצים באש, “he shall burn it on wood in fire.” You will note that the Torah describes the process of burning before mentioning fire, the reverse of what is written in our verse here. It is therefore likely that the penalty of death by burning applicable to the daughter of the priest mentioned in our verse results in injuries similar to those suffered by the sons of Aaron when heavenly fire entered their nostrils. In other words, the life-force, נשמה, is burned whereas the body remains intact.