A definition of history in a dictionary would say something along the lines of:
History:
1. The branch of knowledge dealing with past events.
2. A continuous, systematic telling of past events.
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If, when defining history, we were to only look up a definition in a dictionary, we would have a simplistic view of what this branch of study does. History is definitely concerned with telling past events. However, this information is often incomplete, contradictory, too abundant, too unclear or inexistent. Even when information is readily available, the source of information can never be perfect because of problems that are inherent in recording events, such as perspective, or bias.
The task of historians, then, is to try to be as precise as possible without adding information they cannot be sure about. They also have to draw their own conclusions when there is not enough definite evidence. This is what the paragraph refers to as an argument, because of the interplay of evidence and conclusion.
Therefore, because of all the inherent problems in interpretation, the thing that historians can control the most is the method, or the “way of reading and writing about events of the past.” If, during interpretation, historians make use of a systematic, professional set of rules to interpret, they can be more confident in their conclusions. This set of rules is what people have to master if they desire to become historians.