Jamieson-Fausset-Brown on Numbers 35:22
But if he thrust him suddenly without enmity, or have cast upon him any thing without laying of wait, &c.--Under the excitement of a sudden provocation, or violent passion, an injury might be inflicted issuing in death; and for a person who had thus undesignedly committed slaughter, the Levitical cities offered the benefit of full protection. Once having reached the nearest, for one or other of them was within a day's journey of all parts of the land, he was secure. But he had to "abide in it." His confinement within its walls was a wise and salutary rule, designed to show the sanctity of human blood in God's sight, as well as to protect the manslayer himself, whose presence and intercourse…
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