By way of metaphor, one can describe the centre of the human experience as the being in the chest. And one can distinguish between fundamentally two different modalities, two modalities that you can experience the subject as, the subject being the self or the one that’s experiencing. In Tasawwuf we refer to one modality as nafs and the other modality as ruh.
The natural order for the human being is disturbance and disquiet. The state of the heart outside of dhikr is one of agitation. It is with the dhikr of Allah that the heart becomes tranquil.