Your mental processes can be affected by the pesticides, radiation, drugs and other environmental toxins that can accumulate in your body over time. These things damage not just your health, but your personality and your ability to think clearly. Clear Body, Clear Mind-The Effective Purification Program, by L. Ron Hubbard, details his groundbreaking discoveries in this field. There is a way to eliminate drug and toxic residues lodged in the fatty tissues of your body.
I have seen people who were intellectually vigorous in their seventies, and I have seen people (especially those who took a lot of drugs and medications) who lost a lot of their intellectual vigor long before they retired. I would recommend the Purification Program to anyone who feels the need to recover some of their ability to do highly skilled and creative work.
Bob