Thou wilt keep him in perfect peace. — The italics show that the English version is made up with several interpolated words. More literally, and more impressively, we read, Thou establishest a purpose firm; peace, peace, for in Thee is his trust. Completeness is expressed, as elsewhere, in the form of iteration. No adjectives can add to the fulness of the meaning of the noun.
Related Commentaries of Isaiah 26:3
Isaiah 26:3
3 Thou wilt keep him in perfectb peace, whose mind is stayed on thee: because he trusteth in thee.