Nothing is more important than guarding the mind. Let us constantly keep watch over the wild elephant of the mind, curbing it with mindfulness and vigilance. This is how to avoid being influenced by different external conditions. But even in retreat in a very secluded place, if the mind is not kept under control, it will wander all over the place. Even completely alone, we can have an enormous amount of negative emotions.
How are we to guard the mind? We should use attentiveness to watch our thoughts and use mindfulness to judge whether we are acting correctly. With these two we have the means to annihilate all adverse conditions. But without them, we will not see whether our thoughts are positive or negative or whether we are doing right or wrong, nor will we then be able to use antidotes as necessary.
From For the Benefit of All Beings, page 40.