The fundamental principle of Quaker theology is found in the doctrine of the inward light; and to the exposition and establishment of that doctrine, William Penn devotes himself in his work, entitled _The Christian a Quaker_ (1674). He explains the light as being not something metaphorical, nor yet the mere spirit or reason of man, but Christ, “that glorious Sun of Righteousness and heavenly luminary of the intellectual or invisible world, represented of all outward resemblances, most exactly by the great sun of this sensible and visible world; that as this natural light ariseth upon all, and gives light to all about the affairs of this life, so that Divine light ariseth upon all and gives light to all that will receive the manifestations of it about the concerns of the other life.” That light manifests sin, and reveals duty. It saved from Adam’s day, through the holy patriarchs’ and prophets’ time down to Christ; amongst the Jews as proved from Scripture, amongst the Gentiles, as proved from their own literature. Under this division, Penn quotes largely from the _Stromata_ of Clement of Alexandria, adopting his quotations as genuine and trustworthy. The primitive Fathers expressed themselves in accordance with this doctrine; and amongst the heathen there were men of virtuous lives, who taught the indispensableness of virtue to life eternal. The author contends that the latter foresaw the coming of Christ, and curiously adds, that their refusing to swear proves the sufficiency of the inward light.[500] In the support of these opinions, Penn appeals to the authority of Scripture, and employs a large amount of general reasoning. [Sidenote: QUAKERS.--WILLIAM PENN.]