Physical materials have consistent and absolute scientific properties such as weight, tensile strength, color, melting point, etc. These properties must be considered when building anything. You cannot build a skyscraper out of marshmallows.
All physical properties can be verified through experimentation, which yields the same results if all variables are properly controlled. Experiments need to repeated several times, with consistent results. The results and all the details of the experiments are published in peer-reviewed journals before the results are widely accepted.
People also largely behave in predictable ways, too, both individually and in groups. Experimentation and observation show us how.