Ps 113:16

“Rightly, therefore, does he who is intent on cleansing our heart follow up what He has said with a precept, where He says: "Lay not up for yourselves treasures upon earth, where moth and rust doth corrupt, and where thieves break through and steal: but lay up for yourselves treasures in heaven, where neither moth nor rust doth corrupt, and where thieves do not break through nor steal. For where your treasure is, there will your heart be also." If, therefore, the heart be on earth, i.e. if one perform anything with a heart bent on obtaining earthly advantage, how will that heart be clean which wallows on earth? But if it be in heaven, it will be clean, because whatever things are heavenly are clean. For anything becomes polluted when it is mixed with a nature that is inferior, although not polluted of its kind; for gold is polluted even by pure silver, if it be mixed with it: so also our mind becomes polluted by the desire after earthly things, although the earth itself be pure of its kind and order. But we would not understand heaven in this passage as anything corporeal, because everything corporeal is to be reckoned as earth. For he who lays up treasure for himself in heaven ought to despise the whole world. Hence it is in that heaven of which it is said, "The heaven of heavens is the Lord's," i.e. in the spiritual firmament: for it is not in that which is to pass away that we ought to fix and place our treasure and our heart, but in that which ever abideth; but heaven and earth shall pass away.”