Charles Ellicott Commentary Exodus 20:17

"Thou shalt not covet thy neighbor`s house, thou shalt not covet thy neighbor`s wife, nor his man-servant, nor his maid-servant, nor his ox, nor his ass, nor anything that is thy neighbor`s." — Exodus 20:17 (ASV)

Thou shalt not covet. — This command seems to have been added in order to teach the general principle that the Law of God is concerned, not with acts and words only, but with the thoughts of the heart. Rightly understood, the seventh and eighth commandments contain the tenth, which strikes at covetousness and lustful desire .

But ancient moralists did not usually recognize this; thought, unless carried out into acts, was regarded as “free”; no responsibility was considered to attach to it, and consequently, no one felt it needful to control his thoughts or regulate them. It was therefore of importance that the Divine Law should distinctly assert control over men’s thoughts and feelings, since they are the source of all that is evil in word and act; and true godliness consists in bringing every thought into captivity to Christ (2 Corinthians 10:5).