An acceptable level of mercury in the blood is less than 10 micrograms/liter. It isn't zero. In fact, for many toxic chemicals — lead, arsenic, pesticides — the acceptable level isn't zero. Instead, it's a finite, measurable number, however small. The same is true for the toxic thoughts of the mind.
It's unlikely that your mind will have no anger, no hatred, no envy and no falsehoods. These, too, are toxins. You can try to purge your mind of these toxins, but they will still manage to creep in from some unguarded corner. As you get older, you develop wrinkles. As you live, learn and get hurt, you get scars — emotional scars. Being alive means being imperfect, and this extends to the thoughts in your mind.
Imperfections can serve as an alert, reminding you that people who struggle with anger, hatred, envy or falsehood may have been exposed to a higher dose of it earlier in their lives. Recognizing their constraints, do your best to have compassion for their previous suffering, which has led them to be who they are today. Also try to be compassionate as you reflect on the hurt that is causing their negativity, even if they don't know or feel it.
The imperfections I see in others teach me about my own predispositions. It is my hope that working to rid my mind of anger, hatred, envy and falsehood will help me develop greater compassion toward those who experience these feelings in the world around me. In the end, my hope is that all I will see are individual beings, struggling with predispositions that prevent them from being consistently kind, who are slowly but surely working their way toward freedom.
Thinking in this way can make us all a little kinder. And it is this instinctive kindness that can empower each one of us to make a little corner of the world happier than it would have been otherwise.