Whatever then can help to guide us to this object; viz., purity of heart, we must follow with all our might, but whatever hinders us from it, we must shun as a dangerous and hurtful thing. For, for this we do and endure all things, for this we make light of our kinsfolk, our country, honors, riches, the delights of this world, and all kinds of pleasures, namely in order that we may retain a lasting purity of heart. And so when this object is set before us, we shall always direct our actions and thoughts straight towards the attainment of it; for if it be not constantly: fixed before our eyes, it will not only make all our toils vain and useless, and force them: to be endured to no purpose and without any reward, but it will also excite all kinds of thoughts opposed to one another. For the mind, which has no fixed point to which it may return, and on which it may chiefly fasten, is sure to rove about from hour to hour and minute to minute in all sorts of wandering: thoughts, and from those things which come to it from outside, to be constantly changed into that state which first offers itself to it.
Cassian’s Conferences, First Conference of Abbot Moses, Chapter 5, NPNF2.11.29