According to this theory, the essential things in history are memory and authority. If an event or a state of things is to be historically known, first of all some one must be acquainted with it; then he must remember it; then he must state his recollection of it in terms intelligible to another; and finally that other must accept the statement as true. History is thus the believing some one else when he says that he remembers something. The believer is the Historian; the person believed is called his authority.
This doctrine implies historical truth, so far as it is at all accessible to the historian, is accessible to him only because it exists ready made in the ready-made statements of his authorities. These statements are to him a sacred text, whose value depends wholly on the unbrokenness of the tradition they represent. He must therefore on no account tamper with them. He must not mutilate them; he must not add to them; and, above all, he must not contradict them. For if he takes it upon himself to pick and choose, to decide that some of his authority’s statements are important and other not, he is going behind his authority’s back and appealing to some other criterion; and this, on the theory, is exactly what he cannot do. if he adds to them, interpolating in them constructions of his own devising, and accepting these constructions as additions to his knowledge, he is believing something for a reason other than the fact that his authority has said it; and this again he has no right to do. Worst of all, if he contradicts them, presuming to decide that his authority has misrepresented the facts, and rejecting his statements as incredible, he is believing the opposite of what he has been told, and committing the worst possible offence against the rules of his craft. The authority may be garrulous, discursive, a gossip and a scandal-monger; he may have overlooked or forgotten or omitted facts; he may have ignorantly or wilfully mis-stated them; but against these defects the historian has no remedy. For him, on the theory, what his authorities tell him is the truth, the whole truth, and nothing but the truth.
These consequences of the common-sense theory have only to be stated in order to be repudiated. Every historian is aware that on occasion he does tamper in all these three ways with what he finds in his authorities. He selects from them what he thinks important, and omits the rest; he interpolates in them things which they do not explicitly say; and he criticizes them by rejecting or amending what he regards as due to misinformation or mendacity. But I ma not sure whether we historians always realize the consequences of what we are doing. In general, when we reflect on our own work, we seem to accept what I have called the common-sense theory, while claiming our own rights of selection, construction, and criticism. No doubt these rights are inconsistent with the theory; but we attempt to soften the contradiction by minimizing the extend to which they are exercised, thinking them of as emergency measures, a kind of revolt into which the historian may be driven at times by the exceptional incompetence of his authorities, but which does not fundamentally disturb the normal peaceful regime in which he placidly believes what he is told because he is told to believe it. Yet these things, however seldom they are done, are either historical crimes or facts fatal to the theory: for on the theory they ought to be done, not rarely, but never. And in fact they are neither criminal nor exceptional…
…no historian, not even the worst, merely copies out his authorities; even if he puts in nothing of his own (which is never really possible), he is always leaving out things which, for one reason or another, he decides that his own work does not need or cannot use. It is he, therefore, and not his authority, that is responsible for what goes in. On that question he is his own master: his thought is to that extent autonomous.
~R.G. Collingwood, The Idea of History
The common sense is that we may only know of things that have gone by by people talking about it–purely historical in nature, though, discounting archeology and art. Nonetheless sources may be flawed and incomplete, so archeology, anthropology and other fields of knowledge supplement to create a complete picture within the historian’s mind.
And of course the unreliability of sources makes assumption that they are authorities problematic. It is an all or nothing proposition. Making a judgment that one source is valid and another is not is not acceptable; if one source is and another isn’t then the historian is already doing his job and making judgments, but in doing so should not take for granted that any source is entirely accurate. Each is a source and in collecting and synthesizing and recreating history in the mind, a historian may recount and explain the subject in a new manner.
For this reason it amused me as entirely impossible to use sources as authorities, they are inherently flawed and must be viewed with suspicion.