This is a Persian proverb meaning that when one has doubts about something, it will lead that person to seek answers. An example of this in science is Charles Darwin having doubts that all humans simply were and had always been as God created them, which led him to theorize natural selection and evolution. An example in literature is Shakespeare's King Henry V. Henry had doubts that he was the rightful king and wasn't sure whether he had divine right. Because of this, he felt he had to prove himself with a victory against France, and he demonstrated that he did have divine right and said, "God fought for us."

However, doubt is neither a requirement nor a guarantee of knowledge acquisition. Knowledge can be gained without doubt, and having doubts does not guarantee that one will acquire knowledge. You should also consider that there are different interpretations of the term "doubt" as well as "knowledge". Doubt can imply being uncertain, or it can imply rejecting an idea before it has been explored. Knowledge can be taken to mean discovery, or it could be extended to mean every superficial detail that is perceived and processed by the brain. So you decide: what exactly is the relationship between doubt and knowledge, and to what extent is the Persian proverb, "Doubt is the key to knowledge" true?