Which means that as a scientist (aka natural philosopher) one must take _as a given_ that:

* Happenings that are observed in the external world are related (cause and effect exist) * The principle of sufficient reason applies to all effects (there must be a cause sufficient to explain the effects in all cases) * Everything is not unique (there are categories and kinds - "all chairs" is a valid statement describing something that exists in reality) * What is observed is related to reality (the external world is knowable)

There's much more that a natural philosopher must take for granted, just as there are many physical facts that biology takes for granted. One cannot prove these using the scientific method, because the scientific method presupposes these principles.