This is exactly what the Bible says in James 1.
You don't have to turn there, but in James 1 it says,
Let no man say when he is tempted, I am tempted of God.
For God cannot be tempted with evil,
neither tempteth he any man,
but every man is tempted when he is drawn away
of his own lust and enticed.
Then when lust hath conceived, it bringeth forth sin,
and sin when it is finished bringeth forth death.
And that's exactly what we see in the story about Achan.
The Bible's real consistent on this,
that all sin begins in our heart,
and it all begins with evil thoughts,
or wrong thoughts proceeding out of our heart.
He said, Lust conceives and brings forth sin,
sin when it is finished bringeth forth death.
The word lust is synonymous with the word covetousness.
Because the Apostle Paul said in Romans chapter seven,
I had not known lust except that the scripture had said,
Thou shalt not covet.
He defined lust as being the committing
of the sin of covetousness.
That's what it is.