Thomas Brooks emphasizes the distinction between God's pure light and the mixed nature of worldly possessions. He explains that while God is entirely free from darkness, earthly pursuits are fraught with troubles and sorrows, making them ultimately vain. Brooks illustrates that even the most seemingly desirable things come with hidden burdens, akin to a crown lined with thorns. He warns that ambition for high places brings not only cares but also the risk of moral and spiritual downfall. The sermon calls believers to seek the unmixed light of God rather than the mixed portions of the world.