Verse Lv13-02 states
When a man shall have in the skin of his flesh a
Sin Aleph Tauv a scab, or bright spot, and it is on the skin of his flesh like the disease of leprosy; then he shall be brought to Aaron the priest, or to one of his sons the pries
Rashi commenting on the meaning of the Hebrew
word Sin-Aleph-Tauv states:
Names of leprosy plagues ...varying degrees of whiteness.
Rashi does not further explain how he knew
this meaning. I would conjecture that Rashi used the
method of meaning by root. Indeed, the Hebrew
root of the word Sin Aleph Tauv is Nun Sin
Alpeh which means (among other things), cloud.
Hence Sin Aleph Tauv would mean cloud-white.
It is common in languages to name colors by related
objects of that color - this technique is known
as metonomy. Some typical examples in English
of colors named by related objects with the
same color are orange, aqua, violet. Similar
examples abound in other languages.
Advanced Rashi: Without the methods
of this email list we might have dismissed this
Rashi as a meaning known through oral tradition.
While Judaism certainly has an oral tradition it also has
a tradition based on logic and inference. It is important
to show logical derivations and inferences when they
apply. Such derivations are supplemental and supportive
or oral traditions and strengthen them.