"The skeptic does not mean him who doubts, but him who investigates or researches, as opposed to him who asserts and thinks that he has found"

In this quote, Miguel de Unamuno is making a distinction in between two types of people: doubters and those who assert and believe they have discovered the reality. A skeptic is not someone who questions, however someone who examines or researches. This suggests that a doubter is open to the possibility that their beliefs may be incorrect and is willing to search for evidence to support or refute them. On the other hand, those who assert and believe they have found the fact are more likely to be obstinate and reluctant to think about any proof that contradicts their beliefs. Unamuno is suggesting that suspicion is a more efficient approach to understanding and reality than merely asserting one's beliefs. He is encouraging individuals to be open up to the possibility that their beliefs might be incorrect and to be ready to investigate and research in order to find the fact.