And if thou wilt make me an altar of stone—i.e; if, notwithstanding my preference expressed for an altar of earth, thou wilt insist on making me one of stone, as more permanent, and so more honourable, then I require that the stones shall be rough stones shaped by nature, not stones chiselled into shape by the art of man.
For if thou lift up thy tool upon it thou hast polluted it. It is conjectured with reason that we have here an old traditional idea, which God thought fit under the existing circumstances to sanction.
The real object was that altars should not be elaborately carved with objects that might superinduce idolatry. The widely prevalent notion, that nature is sacred, and that all man's interference with nature is a defilement, was made use of economically, to produce the desired result.
No tool being allowed to be used, no forms of living creatures could be engraved, and so no idolatry of them could grow up.