Jamieson, Fausset and Brown on Isaiah 26:3

3. mind ... stayed--(Psa 112:7, 8). Jesus can create "perfect peace" within thy mind, though storms of trial rage without (Is 57:19; Mr 4:39); as a city kept securely by a strong garrison within, though besieged without (so Php 4:7). "Keep," literally, "guard as with a garrison." Horsley translates, (God's) workmanship (the Hebrew does not probably mean "mind," but "a thing formed," Ep 2:10), so constantly "supported"; or else "formed and supported (by Thee) Thou shalt preserve (it, namely, the righteous nation) in perpetual peace."