Not to be able to stop thinking is a dreadful affliction, but we don't realize this because almost everyone is suffering from it, so it is considered normal. This incessant mental noise prevents you from finding that realm of inner stillness that is inseparable from Being.
The philosopher Descartes believed that he had found the most fundamental truth when he made his famous statement: "I think, therefore I am." He had, in fact, given expression to the most basic error: to equate thinking with Being and identity with thinking.
Source: The Power of Now : A Guide to Spiritual Enlightenment, Pages: 15