Jayarava did a blog article on this one some time ago and concluded it was not from the Buddha. His exposition is rather long, but worth reading. I agree with him, by the way.
The closest I know of to this quote is in Majjhima Nikaya 19, “Whatever a monk keeps pursuing with his thinking and pondering, that becomes the inclination of his awareness.” That’s a rather different statement, of course.
“What you think, you become” has always puzzled me. If I think about Lady Gaga I’m not going to become an outré pop star. But that’s probably just me being literalist. I suppose it’s intended to mean something like “Whatever a monk keeps pursuing with his thinking and pondering, that becomes the inclination of his awareness.”
Here’s a fuller version of that quote:
Whatever a monk keeps pursuing with his thinking and 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.
This is from a sutta called the Dvedhavitakka, or “Two Modes of Thinking,” where the Buddha is talking about his realization, before his Enlightenment, that there were two tendencies within the mind.
First, he would notice that, ‘Thinking imbued with sensuality [or ill will, or harmfulness] has arisen in me; and that leads to my own affliction or to the affliction of others or to the affliction of both. It obstructs discernment, promotes vexation, and does not lead to Nibbana.’
He further noticed that as he mindfully observed this kind of thinking, with an awareness that it led to suffering, it would subside.
Second, he would notice that ‘Thinking imbued with renunciation [and non ill will, and non-harmfulness] has arisen in me; and that leads neither to my own affliction, nor to the affliction of others, nor to the affliction of both. It fosters discernment, promotes lack of vexation, and leads to Nibbana.’
And having observed the arising of this kind of thinking, he would give it his mindful attention. As he says, in a rather lovely simile:
Just as in the last month of the hot season, when all the crops have been gathered into the village, a cowherd would look after his cows: While resting under the shade of a tree or out in the open, he simply keeps himself mindful of ‘those cows.’ In the same way, I simply kept myself mindful of ‘those mental qualities.’
From that point on, to cut a long story short, he entered the jhānas and then got enlightened.
So this is the context of “Whatever a monk keeps pursuing with his thinking and pondering, that becomes the inclination of his awareness.” It means that the mind is trainable, and what kind of thoughts we put our energy into come to shape the mind, and affect both its affective tone (are we happy or unhappy) and its ability to discern the truth.
It’s been suggested that the “what you think, you become” quote may also stem from the first two verses of the Dhammapada, which express in poetic form what the Dvedhavitakka Sutta explains in a more expanded form:
1. Mind precedes all mental states. Mind is their chief; they are all mind-wrought. If with an impure mind a person speaks or acts suffering follows him like the wheel that follows the foot of the ox.
2. Mind precedes all mental states. Mind is their chief; they are all mind-wrought. If with a pure mind a person speaks or acts happiness follows him like his never-departing shadow.
These verses are from the “Chapter on the Pairs” (Yamakavagga) which explores these two modes of thinking, or being.
This derivation, rather than the Dvedhavitakka Sutta origin, may be supported by the fact that “What you think, you become” is often seen in another form: “The mind is everything; What you think, you become.” The connection may not be obvious, but sometimes those Dhammapada verses have been translated to include “our life is the creation of our mind” rather than “our mind is the creation of our thoughts.” And it’s not a great leap from “our life is the creation of our mind” to “the mind is everything.” So that may be the origin of this suspect quote.
Eknath Easwaran’s translation of the first verse of the Dhammapada in fact begins, “Our life is shaped by our mind, for we become what we think.” This is not at all far from “The mind is everything; What you think, you become.”
And that fuller version of the quote is very old indeed. I’ve found it in a 1897 book, In Tune with the Infinite, by Ralph Waldo Trine. Trine used “The mind is everything; What you think, you become” in several of his books, but I haven’t been able to establish where he got it from. I’ll keep looking.
These two Dhammapada verses are often rendered in a very different way from how they were intended, along the lines of “The world is the creation of your mind” — but that’s for another fake Buddha Quote post.
PS A close match is from a non-Buddhist source: ‘You are what you think’ (from the 11th verse of Chapter 1 of the ‘Ashtavakra Gita’ – Marshall’s translation, 2005).
Also, Easwaran’s translation of Dhammapada verse 1 has “Our life is shaped by our mind; we become what we think.” However, the first line of Dhammapada 1 is Manopubbaṅgamā dhammā manoseṭṭhā manomayā.
I would translate this as “All experiences (dhammā) are preceded by mind (Manopubbaṅgamā), having mind as their master (manoseṭṭhā) created by mind (manomayā).”
I don’t see anything in there that corresponds to “we become what we think.”