“He who does anything because it is the custom, makes no choice. He gains no practice either in discerning or in desiring what is best. The mental and moral, like the muscular powers, are improved only by being used. The faculties are called into no exercise by doing a thing merely because others do it, no more than by believing a thing only because others believe it. If the grounds of an opinion are not conclusive to the person's own reason, his reason cannot be strengthened, but is likely to be weakened by his adopting it: and if the inducements to an act are not such as are consentaneous to his own feelings and character (where affection, or the rights of others, are not concerned), it is so much done towards rendering his feelings and character inert and torpid, instead of active and energetic.”
-From “On Liberty” book by John Stuart Mill
In which environment are we supposed to be practicing our mental faculties?
We haven't really built a society where intellectualism is encouraged. School is about memorization, work is doing what we're told and the home is where we shut our brain and do as little thinking as possible.
"...a mind that feeds only on itself soon is undernourished, becoming weak and incapable of creative progressive thought. Stimulation from others is excellent mind food."
- David J. Scwartz
For someone that reads on a regular basis, it can be difficult to find stimulation in the conversation with non-readers. As I mentioned before, there's a thinking distance phenomenon felt during discussions, with a non-reader, which is made increasingly evident the more books someone has read (since the distance is now greater). Stimulating discussion, to an intellectual, are sadly few and far in-between.
Again, the question is how are we suppose to practice our mental faculties?
While we may not be able to change our work situation, educational system or relations - we can choose how we spend our free time. If we expose ourselves to new experiences, or books, it should help with our mental abilities. Writing is likely the best thing we can do to develop our mental capabilities.
As John Stuart Mill said, the brain is a muscle that needs to be trained and it falls on us to insure that we give it proper training.