It's straight from the famous Dhammapada verse "Not to do any evil, to do what is good, and to purify the mind: this is the teaching of the Buddhas."
My sig is my interpretation of that verse. "'No' to the negative" is "Not to do any evil". "'Yes' to the positive" is both "To do what is good" and "To purify the mind". Purifying the mind is part of doing what is good.
I have verified that in my own experience. Whenever I have done good acts such as generosity or kindness, I've felt good about that. Whenever I've done bad kamma - that just sucked. Whenever I've felt happy and blissed out - that was awesome! Whenever I've experienced pain - that sucked.
So it's a well thought out philosophy. One just can't argue with it. I'm not perfect and I may make mistakes from time to time, but I am trying my best to do what is right and not to do what is wrong. I don't sweat it if I make harmless mistakes, but I do avoid doing bad kamma.
Do not think lightly of evil, saying, "It will not come to me." A drop at a time is the water pot filled. Likewise, the fool, gathering it little by little, fills himself with evil.
Do not think lightly of good, saying, "It will not come to me." A drop at a time is the water pot filled. Likewise, the wise man, gathering it little by little, fills himself with good.
—Gotama Buddha, Dhammapada 121-122