The fifth aid is the one from the suttas where mindfulness of breathing is discussed.3 That aid is to notice the long breaths and the short breaths. Pay attention to each in-breath and notice if the current one is longer than average or shorter than average. Do the same for each out-breath. You can also pay attention to see if every long in-breath is always followed by a long out-breath—and the same for short in- and out-breaths.
