This quote, "Just go on . . . and faith will soon return. To a friend hesitant with respect to infinitesimals," speaks to the idea of perseverance in the face of uncertainty or doubt. Infinitesimals, in mathematical terms, refer to quantities so small that they are close to zero, but not quite zero. In this context, d’Alembert is encouraging his friend to continue on, even when dealing with the smallest, seemingly insignificant components.

The concept of infinitesimals can be daunting because it challenges our conventional understanding of size and significance. It suggests that even the smallest thing can have a profound effect. This can be overwhelming and create hesitation or doubt, as it did for d’Alembert’s friend.

However, d’Alembert’s advice is to simply "go on." He suggests that by continuing to engage with the concept, even if it’s difficult or confusing at first, understanding and faith in the process will eventually return. This is a testament to the power of persistence and the belief that complex ideas can be mastered over time.

This idea can be applied in today’s world in many ways. For instance, in personal development, one might face a challenging task that seems overwhelming at first. Instead of giving up, one can take d’Alembert’s advice and just go on, tackling the task bit by bit. Over time, the task will become less daunting and confidence will grow.

In the context of societal issues, this quote can be applied to the small actions individuals take to contribute to bigger causes. For instance, the small act of recycling or using less water may seem insignificant on its own, but when many people do it, the cumulative effect can be significant.

In conclusion, d’Alembert’s quote offers a valuable lesson in perseverance, patience, and the belief in the significance of even the smallest actions or ideas. Whether in personal development or in contributing to societal issues, the key is to continue on, no matter how insignificant our actions may seem at first.