"Apology For The True Christian Divinity" In Truth triumphant
(Volume 2) Barclay, Robert. 

Light of nature, the errors of the Socinians and Pelagians, who exalt this light, are rejected, saving light; Is universal; it is in all. It is a spiritual and heavenly principle. It is a substance, not an accident. It is supernatural and sufficient. It is the gospel preached in every creature. It is the word nigh in the mouth and in the heart. It is the ingrafted word, able to save the soul. Testimonies of Augustine and Buchanan concerning this light. It is not any part of nature or relics of the light remaining in Adam after the fall. It is distinguished from the conscience. It is not a common gift, as the heat of the fire, and outward light of the sun, as a certain preacher said. It may be resisted. By this light or seed , grace and word of God, he invites all, and calls them to salvation. None of those to whom the history of Christ is preached are saved, but by the inward operation of this light. It is small in the first manifestation, but it groweth. It is slighted by the Calvinists, Papists, Socinians, and Arminians. None can put it to silence.