The Four Noble Truths are taught in Buddhism as the fundamental insight or enlightenment of Sakyamuni Buddha (the historical Buddha), which led to the formulation of the Buddhist philosophy.
1. There is dukkha or suffering.
2. This suffering is caused by the attachment to desire (tanha).
3. There is a way out of suffering.
4. The way out of suffering is to follow the Noble Eightfold Path.
This outline form is exactly that used by doctors of the Buddha's culture when diagnosing and prescribing for a disease: Identify the disease, its cause, whether it is curable, and the prescribed cure. Thus the Buddha treats suffering as a "disease" we can confidently expect to cure.
Because of its focus on suffering, Buddhism is often called pessimistic. But since Gautama Buddha presented a cure, Buddhists consider it neither pessimistic nor optimistic but realistic.