Harden not your hearts - What is Hardness of Heart?

From Gouge's Commentary on Hebrews "In that hardness of heart arises from one's self, even from his own wilfulness, It nearly concerns us to be the more watchful over ourselves, and to withstand the very beginning of hardness. For Satan is very subtle, and seeks to beguile a man by degrees ; and sin is deceitful and of a bewitching nature. It soaks into a man insensibly, and we of ourselves are very foolish, like the silly fish that with a fair bait is soon taken. Hence it is that from small beginnings many come to this high pitch, even to be hardened in heart."