[Avodah] taamei ha mitzvot...what is there mesorah??

[Harvey Benton]

i was reading an excerpt (sent in the mail pesach i think) from the
Rambam's taamei Hamitzvot frm Morei Nevuchim........for kilayim on
animals, (eg ox and donkey) he states that the reason is that they should
not come to inter- mingle (cross-breed) while, i remember Rashi (another
Rishon) bringing a medrash perhaps, stating the reason is because they
move at different paces and thus it is not fair to either one of them....

--How are we (mere humans) to come up with the Taamei Hamitzvot unless
there is a valid mesorah as to such???
HB (eg, dont argue with rishonim??)

hb