The opening lines of the Dhammapada, the collected sayings of the Buddha, read:
We are what we think. All that we are arises with our thoughts. With our thoughts we make the world. Speak or act with an impure mind and trouble will follow you as the wheel follows the ox that draws the cart… Speak or act with a pure mind and happiness will follow you as your shadow, unshakable.
Speaking and acting are the two ways one uses one’s mind to draw trouble or happiness from the world. This fits with Merleau-Ponty and Wittgenstein, who said that when we speak, our words are our thoughts, with no separation between speaking and thinking. The same applies to acts. Perhaps all thinking is rooted in speaking and acting. Perhaps picturing something in the head is rooted in the experience of looking, moving one’s eyes, head, neck and body such that a thing comes into view.
For Buddha, the thinking is the coming into existence of the thinker, such that there is no thinker without thought making it so. The thinking causes the thinker to be a particular thing.
For Descartes, the thinking is evidence of the thinker, leading to the conclusion that there exists a thinker prior to and independent of the thinking.