Jesus read their minds, and said-
Why do you cherish wicked thoughts in your hearts?
Although Our Lord had a specific audience when He asked this question it is as apposite for us now as it was for them then. There is no record of them attempting an answer but I think it is possible for us to do so. The key words to think about, it seems to me, are 'cherish' and 'hearts'
The thoughts which we hold in the fully conscious part of our minds are open to be comprehensively analysed by our Reason. That is, we can examine them from all angles and consider their rightness or wrongness, their fitness for purpose. New ideas are easy to treat in this fashion, older ideas, which have become habitual, are more difficult to see fully because we have ceased being aware that they are ideas. Nonetheless, even with them, searching self-examination allows for their flaws to be spotted and corrected if we have the courage or imagination to do so.
This is not so with the thoughts of the heart. These exist in part below the level of our awareness, we know the conclusions but the process that led to those conclusions being reached is hidden from us. They cannot then be fully examined by Reason but they can powerfully influence what we do and say. The concealed root for many of our heart-thoughts is what Buddhists call desire and Christians lust. Although this last word is usually associated with sex nowadays it really means a strong want or longing for something attainable in this material world. When our lust for something is attached to our will so that we both desire and seek to obtain that something then our heart gives birth to its thoughts.
We can be said to cherish them when we hug them close to ourselves despite the warning which we receive from Reason. Which is to say, because our heart-thoughts are linked to an insatiable lust and a fixed will they constantly present themselves before us (and others) as the wellspring for our actions in the world. To the extent that they do so we can apply our Reason to them and notice their wicked origins and outcomes. Yet Reason alone is powerless to defeat them because, as the Christ noted, we cherish them so.
Only the gift of God's grace through Jesus gives us the strength to defeat the thoughts of the heart. Alone we lack the strength though we may possess the desire. For most of us the last heart-thought to which we most stubbornly cling is the pride that imagines we can win our own battles with ourselves. Once we have the humility and realistic self-understanding to let go of that and allow the Holy Spirit to do His gracious work in us then we can hope to be set free from bondage to sin.
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The picture is Bathsheba bathing from a Book of Hours in the Morgan Library