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Making the mind strong and making the body strong are not the same. To make the body strong we have to exercise it, to push it, in order to make it strong, but to make the mind strong means to make it peaceful, not to go thinking of this and that. For most of us the mind has never been peaceful, it has never had the energy of samādhi, so we must establish it within a boundary. We sit in meditation, staying with the “one who knows.”

~ Ajahn Chah.