Einstein wrote this metaphor in a letter to his son Eduard in 1930. Drawing from his understanding of both physics and human nature, he uses this simple analogy to convey a profound truth about life and progress. The quote reflects his belief that stagnation leads to instability, while continuous forward movement - whether in learning, personal growth, or scientific discovery - is essential for maintaining balance and purpose in life. This insight came from his own experience of constantly pushing boundaries in both his scientific work and personal philosophy.