If we would truly prostrate our souls before God, we must use our bodies to postures of lowliness. If we desire true fervours of devotion, we must make prayer the frequent labor of our lips. If we would banish all pride and passion from our hearts, we must force ourselves to all outward actions of patience and meekness. If we would feel inward motions of joy and delight in God, we must practice all the outward acts of it and make our voices call upon our hearts.
Now, therefore, you may plainly see the reason and necessity of singing psalms. It is because outward actions are necessary to support inward tempers and, therefore, the outward act of joy is necessary to raise and support the inward joy of the mind. – William Law (1686-1761), from A Serious Call to a Devout and Holy Life (1728)