A corrupt heart was the source of all

-- Thomas Boston, "Human Nature in its Fourfold State"

We have seen what man was, as God made him--a lovely
and happy creature. Let us view him now as he has unmade
himself--we shall see him a sinful and a miserable creature.
This is the sad state we are brought into by the fall. Man's
nature is now wholly corrupted. There is a sad alteration,
an astonishing overturning in the nature of man--where,
at first, there was nothing evil--now there is nothing good.

"And God saw that the wickedness of man was great in
the earth, and that every imagination of the thoughts
of his heart was only evil continually." Genesis 6:5

All their wicked practices are here traced to the fountain and
spring-head--a corrupt heart was the source of all. The
soul, which was made upright in all its faculties--is now wholly
disordered. The heart, which was made according to God's own
heart--is now the reverse of it, a forge of evil imaginations,
a sink of inordinate affections, and a storehouse of all impiety,
Mark 7:21, 22. Behold the heart of the natural man, as it is
opened in our text. The mind is defiled; the thoughts of
the heart are evil; the will and affections are defiled. The
imagination of the thoughts of the heart, that is, whatever
the heart frames within itself by thinking, such as judgment,
choice, purposes, devices, desires, every inward motion--is
evil. Yes, and every imagination, every frame of his thoughts,
is evil.

But is there not, at least, a mixture of good in them? No,
they are only evil. Whatever changes may be found in them,
are only from evil to evil; for the imagination of the heart,
or frame of thoughts in natural men, is evil continually. Not
one holy thought can ever be produced by an unholy heart.

O, what a vile heart is this! O, what a corrupt nature is this!
What can that heart be, whereof every imagination, every set
of thoughts--is only evil, and that continually? Surely that
corruption is ingrained in our hearts, interwoven with our very
natures, has sunk deep into our souls, and will never be cured
but by a miracle of grace. Now such is man's heart, such is his
nature--until regenerating grace changes it.