In Parah 11:7, we learn that hyssop that is called simply "hyssop" (אזוב) is valid for use in sprinkling mei ḥatat, but hyssop that has an accompanying name (such as "blue hyssop", etc.) is unfit.

Is this a descriptive rule based on the language they had at the time? Or possibly, we require something called "hyssop", and if the particular plant we call "hyssop" changes over time, then the acceptable species also changes over time?

(This question can also be asked for several other cases, such as which straight metal utensils are susceptible to tumah, see Keilim 11:2.)

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