This quote by Leonardo da Vinci is a profound reflection on the importance of living in the present and not deferring our enjoyment of life. It suggests that a person of "supreme folly" is one who is always looking forward to the future, hoping for better times, and in doing so, allows the present moment, and thus their life, to slip away unnoticed and unappreciated.

In essence, da Vinci is warning against the dangers of always living in anticipation, always waiting for something better to come along, rather than appreciating and making the most of the present moment. He suggests that this is a foolish way to live, as it results in a life that flies away unenjoyed.

Applying this idea to today’s world, it could be seen as a critique of our fast-paced, future-oriented society. Many people are always looking ahead, always planning, always striving for more – more money, more success, more happiness. But in doing so, they may miss out on the joy and beauty of the present moment. They may end up living their lives in a state of constant anticipation, rather than truly experiencing and appreciating each day.

In terms of personal development, this quote could be a reminder to practice mindfulness and gratitude. Mindfulness involves being fully present and engaged in the current moment, rather than constantly thinking about the past or future. Gratitude involves appreciating what we have right now, rather than always longing for more. By practicing these skills, we can avoid the "supreme folly" da Vinci warns against and live a more fulfilling and enjoyable life.