For if any does a work with the mind of gaining thereby an earthly good, how will his heart be pure while it is thus walking on earth? For any thing that is mingled with an inferior nature is polluted therewith, though that inferior be in its kind pure. Thus gold is alloyed when mixed with pure silver; and in like manner our mind is defiled by lust of earthly things, though earth is in its own kind pure.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: Serm. in Mont., ii, 13