When you’re not doing something good, then automatically the mind is thinking of something unwholesome – some delusion, craving, aversion, anxiety, or worry will be operating. 

That is why we should take care of the mind; we keep it busy with the wholesome. In mindfulness meditation, we do not let the mind go idle. We always give it a wholesome job, moment to moment.

If you’re not doing something good, you are already doing something bad. The puthujjana (worldling) mind is either wholesome or unwholesome.