John Locke - Principles

It is not unusual to see men rest their opinions upon foundations that have no more certainty and solidity than propositions built on them and embraced for their sake. Such foundations are these and the like, viz. the founders or leaders of my party are good men, and therefore their tenets are true; it is the opinion of a sect that is erroneous therefore it is false; it has been long received in the world, therefore it is true; or it is new, and therefore false.

We are born to be, if we please, rational creatures, but it is use and exercise only that makes us so, and we are indeed so no farther than industry and application carried us.

John Locke - Of Practice and Habits

...but it is only the exercise of those powers which gives us ability and skill in anything and leads us towards perfection. 

I do not deny that natural disposition may often give the first rise to it; but that never carries a man far without use and exercise and it is practice alone that brings the powers of the mind as  well as those of the body to their perfection.