So when we say that covetousness is the root of all evils, we do not assert that no other evil can be its root, but that other evils more frequently arise therefrom, for the reason given.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic evil reason
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/17897/pg17897-images.html

Context

“3: Just as in natural things we do not ask what always happens, but what happens most frequently, for the reason that the nature of corruptible things can be hindered, so as not always to act in the same way; so also in moral matters, we consider what happens in the majority of cases, not what happens invariably, for the reason that the will does not act of necessity. So when we say that covetousness is the root of all evils, we do not assert that no other evil can be its root, but that other evils more frequently arise therefrom, for the reason given. ________________________
SECOND ARTICLE [I-II, Q. 84, Art. 2]
Whether Pride Is the Beginning of Every Sin?
Objection 1: It would seem that pride is not the beginning of every sin. For the root is a beginning of a tree, so that the beginning of a sin seems to be the same as the root of sin.” source