How do we know the concept of vanity in man's life is true?

The idea that human endeavors are vanity is supported throughout scripture, particularly in Ecclesiastes where Solomon explores the futility of chasing after earthly pleasures and achievements. He concludes that all is vanity. In Psalm 39, David expresses a similar sentiment, emphasizing that even at our best, our lives are transient and ultimately insignificant compared to God's eternal nature. This truth serves to humble us and draw our focus away from temporal pursuits towards a reliance on God's glory and grace for true fulfillment.