“All that we are is the result of what we have thought. The mind is everything. What we think, we become.”
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Maharishi Mahesh Yogi 17
Inventor of Transcendental Meditation, musician 1917–2008Related quotes

“We are what we think.
All that we are arises with our thoughts.
With our thoughts we make the world.”
As rendered by T. Byrom (1993), Shambhala Publications.
There is no quote from the Pali Canon that matches up with any of these. The closest quote to this is in the Majjhima Nikaya 19:
"Whatever a monk keeps pursuing with his thinking & pondering, that becomes the inclination of his awareness. If a monk keeps pursuing thinking imbued with sensuality, abandoning thinking imbued with renunciation, his mind is bent by that thinking imbued with sensuality. If a monk keeps pursuing thinking imbued with ill will, abandoning thinking imbued with non-ill will, his mind is bent by that thinking imbued with ill will. If a monk keeps pursuing thinking imbued with harmfulness, abandoning thinking imbued with harmlessness, his mind is bent by that thinking imbued with harmfulness." Sources: http://www.accesstoinsight.org/tipitaka/mn/mn.019.than.html
Misattributed

Talking about Kabbalah http://www.thelpa.com/lpa/quotes.html

“We become, neurologically, what we think."(33)”
Source: The Shallows: What the Internet is Doing to Our Brains