Argumentation is the process of working from shared knowledge or common assumptions, to conclusions that are not shared or held in common.
In argumentation, virtually all of the premises on which we rely are based on authority -- that is, they are supplied by others, not by ourselves. We simply lack the expertise or the experience to know everything of which we speak. I doubt whether we are the ultimate source of even a fraction of our truth claims: only claims of personal experience can suffice.
Even if I were to carry out an entirely independent experiment, I still rely on the education I received from experts in the field to corroborate that I used the proper technique to gather data and reach conclusions; I must trust that others have or will have independently reached the same conclusions and verified my findings; and I must interpret my data and my conclusions in light of the knowledge I share and have received from other scientists.
Human knowledge cannot be objective, for we are fallible humans. Nor, contra Michael Polanyi, is human knowledge an essentially personal activity, though there is an intrinsic human element to all of our intellectual pursuits.
Human knowledge is a fundamentally social endeavor, for all of our efforts to discern truth will rely considerably, if not entirely, on the trust we place in others.
The question of authority -- the extent to which we are comfortable relying on things or persons outside ourselves-- is central to almost every truth claim to which we turn our minds.