John Gill

And Joshua said unto all the people,.... The chief of them now gathered together, and who represented the whole body: behold, this stone shall be a witness unto us; of the covenant now made, and the agreement entered into, as the heap of stones were between Jacob and Laban, Gen 31:45, for it hath heard all the words of the Lord which he spake unto us; this is said by a figure called "prosopopaeia", frequent in Scripture, by which inanimate creatures are represented as hearing, seeing, and speaking, and may signify, that should the Israelites break this covenant, and disobey the commands of the Lord they had promised to keep, they would be as stupid and senseless as this stone, or more so, which would rise in judgment against them. Nachmanides (d) a Jewish commentator, interprets this stone of the Messiah, the same as in Gen 49:24, it shall be therefore a witness unto you, lest ye deny your God; for a memorial and testimony to prevent them from going into atheism, a denying of the true God, or into apostasy from him, and into idolatry and false worship. The Targum of which is,"behold, this stone shall be to us as the two tables of stone of the covenant, for we made it for a testimony; for the words which are written upon it are the sum of all the words of the Lord which he spake unto us, and it shall be unto you for a memorial, and for a testimony, lest ye lie before the Lord.'' (d) Apud Masium in loc.