Here’s a quote from Isaac Watt’s book The Improvement of the Mind. (It’s a shame that Watts is usually known only for his hymn writing.)

NO man is obliged to learn and know every thing; this can neither be sought nor required, for it  is utterly impossible: yet all persons are under some obligation to improve their own understanding; otherwise it will be a barren desert, or a forest overgrown with weeds and brambles. Universal ignorance or infinite errors will overspread the mind, which is utterly neglected, and lies without any cultivation.

Skill in the sciences Is indeed the business and profession but of a small part of mankind; but there are many others placed in such an exalted rank in the world, as allows them much leisure and large opportunities to cultivate their reason, and to beautify and enrich their minds with various knowledge. Even the lower orders of men have particular callings in life, wherein they ought to acquire a just degree of skill; and this is not to be done well, without thinking and reasoning about them.

I’ve found Watt’s Improvement to be an encouraging read, challenging me to fight off the weeds and brambles in my own mind. (I printed out this abriged edition for the clarity of the scanned images.) But don’t take my word on this book – take Samuel Johnson’s:

Few books have been perused by me with greater pleasure than (Watts’) Improvement of the Mind, of which the radical principles may indeed be found in Locke’s Conduct of the Understanding; but they are so expanded and ramified by Watts, as to confer on him the merit of a work in the highest degree useful and pleasing. Whoever has the care of instructing others, may be charged with deficiency in his duty, if this book is not recommended. (Quoted in the preface of Improvement)