This text tries to show the philosophical meaning of the uncertainty which was always linked to the restless researcher Miguel de Unamuno. The deep religious and intellectual crisis, that led him to lose the childhood faith, far from assuming a sceptic and passive attitude, was his starting point of seeking a new existential sense. And, always encouraged by his fighting spirit, the uncertainty about eternal life of individual awareness, became for him the most effective encouragement to long for immortality and be worthy of it through a passionately good life. In that uncertainty between logical and heart there is a tragic feeling of men and population life. It is just an uncertainty without solution, so it doesn’t lead him to hopelessness or search withdrawal, but on the contrary it motivates him to invent, poeticize, imagining an exit and devoting his entire life to fight for its accomplishment.