Though the faculties of the mind are improved by exercise,yet they must not be put to a stress beyond their strength.
Quid valeant humeri,quid ferre recusent must be made the measure of everyone's understanding who has a desire not only to perform well but to keep up the vigor of his faculties and not to balk his understanding by what is too hard for it.The mind,by being engaged in a task beyond its strength,like the body strained by lifting at a weight too heavy,has often its force broken and thereby gets an ineptness or an aversion to any vigorous attempt ever after.Asinew cracked seldom recovers its former strength,or at least the tenderness of the sprain remains a good while after and the memory of it longer,and leaves a lasting caution in the man not to put the part quickly again to any robust employment.
So it fares in the mind once jaded by an attempt above its power;it either is disabled for the future or else checks at any vigorous undertaking ever after,at least is very hardly brought to exert its force again on any subject that requires thought and meditation.The understanding should be brought to the difficult and knotty parts of knowledge,that try the strength of thought,and a full bent of the mind by insensible degrees;and in such a gradual proceeding nothing is too hard for it.Nor let it be objected that such a slow progress will never reach the extent of some sciences.
It is not to be imagined how far constancy will carry a man;however,it is better walking slowly in a rugged way than to break a leg and be a cripple.He that begins with the calf may carry the ox;but he that will at first go to take up an ox may so disable himself as not be able to lift a calf after that.When the mind by insensible degrees has brought itself to attention and close thinking,it will be able to cope with difficulties and master them without any prejudice to itself,and then it may go on roundly.Every abstruse problem,every intricate question will not baffle,discourage or break it.But though putting the mind unprepared upon an unusual stress that may discourage or damp it for the future ought to be avoided,yet this must not run it,by an over great shyness of difficulties,into a lazy sauntering about ordinary and obvious things that demand no thought or application.
This debases and enervates the understanding,makes it weak and unfit for labour.This is a sort of hovering about the surface of things without any insight into them or penetration;and when the mind has been once habituated to this lazy recumbency and satisfaction on the obvious surface of things,it is in danger to rest satisfied there and go no deeper,since it cannot do it without pains and digging.He that has for some time accustomed himself to take up with what easily offers itself at first view,has reason to fear he shall never reconcile himself to the fatigue of turning and tumbling things in his mind to discover their more retired and more valuable secrets.
It is not strange that methods of learning which scholars have been accustomed to in their beginning and entrance upon the sciences should influence them all their lives and be settled in their minds by an over-ruling reverence,especially if they be such as universal use has established.Learners must at first be believers,and,their master's rules having once been made axioms to them,it is no wonder they should keep that dignity and by the authority they have once got mislead those who think it sufficient to excuse them if they go out of their way in a well beaten tract.