Lay not up for yourselves treasures upon earth, where moth and rust does corrupt, and where thieves break through and steal:
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Augustine of Hippo
AD 430
If someone does something with the intent of gaining earthly profit, that one’s heart is upon the earth. How can a heart be clean while it is wallowing in the mud? On the other hand, if it be fastened upon heaven it will be clean, for whatever is heavenly is unpolluted. A thing becomes defiled if it is mixed with a baser substance, even though that other substance be not vile in its own nature. Gold, for example, is debased by pure silver if mixed with it. So also is our mind defiled by a desire for the things of earth, although the earth itself is pure in its own class and in its own order. .
Rightly, therefore, does he who is intent on cleansing our heart follow up what He has said with a precept.
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 fi...
Previously he had only said that we must show mercy. Here he also points out how great is the mercy we must show. He says, “Don’t store up treasure.” It would have been impossible to introduce his discourse on disdain for riches without much preparation. So he broke the discourse up into small portions. Having readied the hearer’s mind, he brings up the tougher subject in a way that is plausible. The Gospel of Matthew, Homily
Thus, after He has cast out the disease of vainglory, and not before, He seasonably introduces His discourse of voluntary poverty. For nothing so trains men to be fond of riches, as the fondness for glory. This, for instance, is why men devise those herds of slaves, and that swarm of eunuchs, and their horses with trappings of gold, and their silver tables, and all the rest of it, yet more ridiculous; not to satisfy any wants, nor to enjoy any pleasure, but that they may make a show before the multitude.
Now above He had only said, that we must show mercy; but here He points out also how great mercy we must show, when He says, Lay not up treasure. For it not being possible at the beginning to introduce all at once His discourse on contempt of riches, by reason of the tyranny of the passion, He breaks it up into small portions, and having set free the hearer's mind, instills it therein, so as that it shall become acceptable. Wherefore, you see, He said first, Blessed are the merciful...
Having first cast out the sickness of vainglory by what He said before, now He speaks about non-possessiveness. For men possess more than they need because of vainglory. He shows how unprofitable earthly treasure is—moth and corruption consume food and clothing, and thieves steal gold and silver. Then, anticipating the objection that not all treasure is stolen, the Lord says, "Even if nothing is lost in this manner, are you not wretched for being nailed down by your worries over wealth?" This is why He says, Where your treasure is, there will your heart be also.
