From his earliest poetry, Heaney used formal linguistic terminology to describe his poetic practice. After Seeing Things, from the point when he developed a stress on the visionary, his linguistic analysis increasingly dwelt on the process by which instinctive observation of objects and experiences underlies language: what he called ‘raids on the prearticulate’. Earlier poems like ‘Alphabets’ described this as a process of education; later poems reverse the process so that existing language is taken back to the unarticulated sentiments and physical realities that underlie it. For Heaney the great image for this is Braille by which material figures work backwards by sensory exploration to the things they represent. He uses it as an image from ‘Bog Queen’ in North to 'At the Wellhead' in Seeing Things.