Whenever we study nature and compare observations with other human beings, we need to have a common set of assumptions. Assumptions are things that are accepted as true or certain to happen without proof.
Most of the common set of assumptions is quite often so basic that it is not even declared. We take a lot of very complex interactions for granted, just because we are intensely familiar with them. Apparently, there is at that time of commonly shared observation no reasonable indication yet about a potential variability of specific phenomena. It was a shock when Einstein stated that time was variable relative to speed. Quantum physics introduced an unexpected level of uncertainty about the status of variables. The issue of the particle/wave duality.
Some other assumptions are more specific and need a further declaration. We use the SI system for comparison of measurement; we assume objectivity when there is repeatability. We declare which variables have been assumed to be constant during an experiment. We need to declare the conditions for repeatability of the experiment and the like. Different observers must be able to arrive at the same outcome for the experiment.