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.
I am on the path, however not yet advanced. Any opinions or insights I share are meant entirely for discussion purposes and in cases where people might find them beneficial in whatever way. Since I am not advanced on the Path, I cannot guarantee that what I say will always necessarily be 100% true or in line with the Dhamma. However, having had an extremely interesting life with a wide variety of different (many of them deep) experiences, I hope that anything I share will be of use, provide food for thought, and inspire interesting and beneficial discussions.
Mettā to all!