Establishing by observation that there is just one black swan falsifies "all swans are white". This is an unexceptional and undeniable point. However… - Alan Chalmers

Empiricism and positivism share the common view that scientific knowledge should in some way be derived from the facts arrived at by observation.

Which facts are relevant and which are not relevant to a science will be relative to the current state of development of that science.

Two normal observers viewing the same object from the same place under the same physical circumstances do not necessarily have identical visual experiences, even though the images on their respective retinas may be virtually identical.