We must be on our guard, therefore, against evil desires, for death lies close by the gate of pleasure. Hence the Scripture gives this command:"Go not after your concupiscences" (Eccles. 18:30). So therefore,since the eyes of the Lord observe the good and the evil (Prov. 15:3)and the Lord is always looking down from heaven on the children of earth"to see if there be anyone who understands and seeks God" (Ps. 13[14]:2), and since our deeds are daily, day and night, reported to the Lord by the Angels assigned to us, we must constantly beware, brethren, as the Prophet says in the Psalm, lest at any time God see us falling into evil ways and becoming unprofitable (Ps. 13[14]:3); and lest, having spared us for the present because in His kindness He awaits our reformation, He say to us in the future,"These things you did, and I held My peace" (Ps. 49[50]:21).

Go not after your concupiscences.

Benedict is quoting from the 18th chapter of Ecclesiasticus or Sirach generally categorized within the Apocrypha. In context as translated by the New Revised Standard Version:

Do not follow your base desires, but restrain your appetites.
If you allow your soul to take pleasure in base desire, it will make you the laughingstock of your enemies.

While originally written in Hebrew, Sirach is part of the Greek Septuagint, which was subsequently translated into Latin. It is possible that Benedict only knew the Latin version. It is unlikely he was familiar with the Hebrew original.

I always want to giggle when hearing concupiscence. The sound and structure strike me as a tad ridiculous. It means to strongly desire something. It came to be the principal Church Latin euphemism for physical lust. Lust is how the King James version translates what Benedict refers to as concupiscence.

The Greek is epithymia. In Plato's allegory of the chariot, the two horses are named Thymos (emotion or passion) and Epithymia (desires or aversions). For Plato there is nothing inherently wrong with either of these powerful motivators. But both must be guided by the Charioteer, Logistikon or reason. The key to a good life, according to Plato, is symmetry and balance among the three parts.