As this question was classified under history, it appears to be a question about what historians call historiography, in other words the methods used by historians and within the discipline of history. History, as a disciple, is not like physics or mathematics where it is possible to detach "facts" from the observer. Every written work is an interpretation of events, not events themselves. On the other hand, just because history cannot being a purely objective science, that does not mean that all historical opinion is unreliable, nor does the individual and personal nature of historical viewpoints mean that all historical works are equally biased and worthless. Instead, one needs to consider how the individual viewpoints and experiences of historians contribute to illuminate different aspects of history. For example, when retired generals write about military history or economic scholars about ancient trade, their knowledge of their specific areas contributes insights into why people may have acted in a certain fashion or why certain systems succeeded or failed.
Finally, although any record is that of one piece of evidence, whether physical (as in coinage, buildings, or pottery), literary (as in ancient historians works, personal letters, diaries), etc. generally the least mediated sources are the primary ones, which is why you should always read a variety of primary materials from the period you are studying and examine physical artifacts rather than relying on secondary sources and summaries.