Charles H. Spurgeon on Psalms 15:2

"And speaketh the truth in his heart." Anatomists have observed that the tongue in man is tied with a double string to the heart. And so in truth spoken there is necessary a double agreement of our words. 1. With our heart. That is, to the speaking of truth, it is necessary our words agree with our mind and thoughts about the thing. We must speak as we think, and our tongues must be faithful interpreters of our mind: otherwise we lie, not speaking as we think. So what is truth in itself may be spoken by a man, and yet he be a liar; namely, if he does not think as he speaks. 2. With the thing as it is in itself. Though we think a thing to be so, which is not so, we lie, when we affirm it; because it is not as we say, though we really think it is so. For our mistaken notions of things can never stamp lies to pass current for truths. 2 Thessalonians 2:11. Thomas Boston.