In building foundations it is important to get down to rock or otherwise the shrinkage and expansion due to the summer heat and the winter rains will do damage to the construction. JESUS tells of a good mason who "digged deep, and laid the foundation on a rock" (Luke 6:48). Deep trenches are dug, and filled with stone and lime, and this is allowed to settle all it will. All this being below the surface of the ground is invisible afterwards, and therefore it is considered a lack of courtesy for one man to build upon another man's foundation, as Paul mentions (Romans 15:20). [Manners And Customs of Bible Lands]

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