Charles H. Spurgeon on Psalms 95:10

It is a people that do err in their heart. In err in heart may mean either to err in judgment, or in disposition, intention: for the Hebrew bbl, and after it the Greek kardia, means either animus, judicium, or, mens, cogitatio, desiderium. I understand kardia here, as used according to the Hebrew idiom (in which it is often pleonastic, at least it seems so to us,)so that the phrase imports simply, They always err, i.e. they are continually departing from the right way. —Moses Stuart.