Sforno on Exodus 21:24/ספורנו על שמות כ״א:כ״ד

1עין תחת עין; this is what ought to be the judgment against the offender, if we were to apply the principle of the punishment fitting the crime in all its severity. However, according to tradition only financial compensation is exacted as we cannot accurately measure how to apply the principle of “an eye for an eye” literally.