For as he thinks within himself, so he is.

Proverbs 23:7
The passage in context is:
"6(F)Do not eat the bread of a (G)selfish man, Or desire his delicacies; 7For as he thinks within himself, so he is. He says to you, "Eat and drink!" But (H)his heart is not with you. "
It is the duty of the Christian to think of others and not selfishly. The selfish man thinks inwardly while the man of God thinks outwardly towards God. There is no power from within, only from God to make real personal change.