“Let us consider the vanity of the contrary opinion [distinguishing between Psalms, hymns and spiritual songs], in regard it utterly makes way for will worship, which they seem to be so much against. For first, I am commanded to sing Psalms, Hymns, Songs; The Old and New Testaments speak of no other Psalms than of David and Asaph and of such like inspired persons; and they are called by Christ and his Apostles, but you must not sing them (say they). I ask what Psalms then must you sing? There is no light in Old or new Testament to warrant any other; either you must fancy a Psalm, and say Christ meant this way, when he spake of the Book of the Psalms, and devise a new way of worship out of your own brains, or else sing the Psalms which Christ and His Apostles call Psalms. Besides, how can any man persuade himself, or others, when he sings, that he sings a Psalm, when he doth not sing that which Scripture has only called a Psalm? Or how can any man distinguish, now I sing a Psalm, now a Hymn, now a Song, where there is not one word in the New Testament to distinguish them from one another, or the two latter from the Book of Psalms? If any man from the New Testament can distinguish a Psalm from a Hymn, or a Hymn from an Ode or a Song, or any one from another, but as they borrow it from the Old Testament, he shall be an Oracle.”
by Cuthbert Sidenham, from A Christian, sober and plain exercitation on the two grand practical controversies of these times : infant baptism and singing of psalms (1654), page 188-189 found here
Exclusive Psalmody 