Conduct, as everything else in nature, abhors a vacuum. When we abandon an unskillful habit, we must cultivate a positive one. Otherwise, from our trove of accumulated karmic tendencies, another habit –similar in quality– will replace the one we’ve just given up. This is why persons who give up drinking alcohol often take up smoking, or persons who give up smoking often take up eating sweets or chewing gum.
However, this ‘vacuum effect’ can be used skillfully. When we abandon a negative habit, we open up a space in our behavioral continuum in which a new positive habit can flourish. Similarly, as we take up a new positive conduct, we crowd out or displace a negative habit.
If the old and the new conducts are related in terms of object, method, and circumstance, the process is more powerful. For example, if we give up smoking (which engages the inhalation and the exhalation, utilizes the fingers and the mouth, and is often performed when we are anxious), we can take up mantra recitation at those times when we used to smoke, mounting the mantra on the breath (reciting the mantra silently as we inhale and exhale), and counting the repetitions with a rosary or mala.
Paired vows (one to abstain from a negative action, the other to adopt a contrary positive action) are prevalent in Dharma practice because they complement each other and facilitate adherence. Paired vows are skillful means.