Rembrandt's use of light in his self-portraits has received an abundance of scholarly attention throughout the centuries--and for good reason. His light delights the eye and captivates the mind with its textural quality and dramatic presence. At a time of scientific inquiry and religious reformation that was reshaping the way individuals understood themselves and their relationship to God, Rembrandt's light may carry more intellectual significance than has previously been thought. Looking at Rembrandt's oeuvre of self-portraits chronologically, it is apparent that something happened in his life or in his understanding that caused him to change how he used light. A distinct and consistent shift can be observed in the location and intensity of light to the crown of the forehead. This change indicates that light held particular significance for Rembrandt and that its connection to the head was a signifier with intentional meaning. This meaning could have developed as a result of Rembrandt's exposure to and interest in the contemporary theological and philosophical debates of the seventeenth-century Netherlands, particularly those relating to the physical and eternal nature of the soul stemming from the writings of René Descartes. The relative religious and intellectual freedom of the Dutch Republic provided a safe place for Descartes to publish and defend his metaphysical ideas relating to the nature of the soul and know-ability of God through personal intellectual inquiry. The widespread disturbance to established thought caused by his ideas and methods sped their dissemination into the early seventeenth-century discourse. Rembrandt's associations with the educated elite, particularly Constantijn Huygens and Jan Six, increases the probability that he knew of this new philosophy and had the opportunity to consider its relevance to his own quest for self-knowledge. With his particular emphasis on self-exploration and expression, demonstrated through his prolific oeuvre of self-portraits, and his inclination toward emotive, complex, and interdenominational religious works, it follows that Rembrandt would be eager to embrace Descartes' metaphysics and demonstrate his awareness through his self-portraits. Light on the forehead becomes a metaphor for enlightenment and is the key to reading Rembrandt's late self-portraits through the lens of Cartesian influence.