"Test yourself on mankind. It is something that makes the doubter doubt, the believer believe"

In this quote, Franz Kafka is prompting readers to analyze and evaluate humankind. He recommends that this act of testing oneself on humanity can have a profound impact on one's beliefs and doubts. By observing and analyzing the actions and behaviors of others, one may start to question their own beliefs and values. This can lead to a sense of doubt and uncertainty, along with a deeper understanding of oneself and the world around them. On the other hand, this process can also reinforce one's beliefs and convictions, as they might find recognition and confirmation in the actions of others. Ultimately, Kafka is highlighting the power and complexity of humanity and the impact it can have on our own beliefs and doubts.