There are different degrees of certainty within science

Physics/chemistryThe purest, most repeatable and highest degree of certainty.
Biology/medicineIntroduces complex systems with emergent properties, meaning we cannot always know why something happens, and in different conditions different results may be experienced.
Randomized control studySo-called “clinical prooves” may only represent a small statistically difference.
Epidemiological studiesStudies what we see in the actual world, but can only show correlation, not causation.