This quote emphasizes the importance of critical thinking and evidence-based reasoning. It suggests that no idea, belief, or practice should be accepted blindly just because it has been around for a long time. Instead, everything should be subjected to scrutiny and must stand up to evidence-based examination and testing.

The "ancient" way of thinking or doing mentioned in the quote refers to traditions, long-held beliefs, or established practices. Thoreau is warning against the danger of accepting these without questioning or seeking proof. This is a call for skepticism and a scientific approach to knowledge, where every claim is tested and validated.

Applying this idea in today’s world, we can see its relevance in various areas like politics, science, religion, and even social norms. For instance, in politics, it’s essential to critically examine the claims made by politicians, regardless of their standing or the age of their ideologies. Similarly, in science, a theory’s age or popularity doesn’t make it inherently true; it must be backed by empirical evidence.

In terms of personal development, this quote encourages us to continually question and re-evaluate our beliefs and habits. Just because we’ve been thinking or doing something a certain way for a long time doesn’t mean it’s the best or only way. It urges us to seek evidence for our actions and beliefs, thereby promoting growth, adaptability, and a more nuanced understanding of the world.

In essence, Thoreau’s quote is a timeless reminder of the importance of critical thinking, skepticism, and the pursuit of evidence, regardless of how old or established a belief or practice might be. It is a call to action for each of us to continually question, learn, and grow.