That's 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 is 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 7, "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.