Isaac of Stella, one of the great monastic thinkers of the Middle Ages, was born in England about 1100. He apparently studied in both England and France before entering the monastery of Citeaux. After several years at Stella (L'Étoile) in Poitou, where he became abbot, Isaac attempted to found a monastery on the lonely island of Ré, near La Rochelle, but soon returned to Stella, where he died about 1169. His writings include a treatise on human nature (De Anima), an exposition of the liturgy (De Officio Missae), and fifty-four sermons, preached either at Ré or at Stella. Through the De Spiritu et Anima of Alcher of Clairvaux his psychological theories became widely influential, notably in the Franciscan school of the thirteenth century.

Isaac's mind, schooled in the biblical spirituality of the Cistercians, was steeped in Scripture, and his...