Jamieson, Fausset and Brown on Isaiah 40:28

28. known--by thine own observation and reading of Scripture.

heard--from tradition of the fathers.

everlasting, &c.--These attributes of Jehovah ought to inspire His afflicted people with confidence.

no searching of his understanding--therefore thy cause cannot, as thou sayest, escape His notice; though much in His ways is unsearchable, He cannot err (Job 11:7-9). He is never "faint" or "weary" with having the countless wants of His people ever before Him to attend to.