Tacitus was influenced by gossip, as was much of ancient writing. But science does theorize, putting together data from the empirical world and selecting what it believes is the most probable among rival hypotheses. In geology, for example, it puts together all it has found in its poking around the earth. It puts together what it knows about rain, rivers and erosion in places like the Grand Canyon. Geologists hypothesize that the earth must be older that a few thousand years, and they select this hypothesis over the rival theories put forth with less empirical evidence.
Science is a method for grasping knowledge, but knowledge is always incomplete.
Grasping reality is a matter of making mental associations. Knowledge does not come to us whole and in complete form. Grasping reality is making distinctions as well as associations. Our associations are generalizations. Some generalizations are finer, with more distinctions, than others.