Read not books alone, but men, and amongst them chiefly thyself. If thou find anything questionable there, use the commentary of a severe friend, rather than the gloss of a sweet-lipped flatterer there is more profit in a distasteful truth than in deceitful sweetness.

Enchiridion Institutions, Essays and Maxims, political, moral & divine. Divided into four centuries. By Francis Quarles (ed. 1844)