The scientific attitude of skepticism is based on the belief that

Questions, hypotheses, and ideas should be tested against observable evidence.

Explanation:

Skepticism is extremely important, if not fundamental, in all areas of Science. This attitude is based on the belief that all questions, hypotheses, and ideas should be tested against observable evidence. Moreover, it allows scientists to question and think thoroughly about all possibilities behind a phenomenon, instead of just 'believing' any observation or vague and non-supported reason that explains it.

In addition, it allows researchers to investigate all possibilities and test numerous methodologies to be certain and gather enough evidence to explain this certain phenomenon.