Understanding your own ability to reason or to analyze a problem is the key to effectively producing a research paper or to present the results of your research before an audience.
In all academic fields, there are disagreement about how to interpret data or material, such as manuscripts from some time in the past. While there are absolute truths in the physical sciences, there are still disagreements about conclusions drawn from certain data. In the social sciences, disagreements are very common because what is being interpreted is far more subjective.
In preparing a research paper, you need to be certain that you've read or consulted enough different sources of information to have more than one side of the issue. And the sources, at least some of them, need to be what are called "primary," meaning they are the first-hand observations of a scholar, scientist, policy-maker, or whatever type of individual is actively working in the field about which you are writing. For example, if you are working on a paper on demographics, then census data derived from first-hand surveys conducted by the government is essential.
When using "secondary" sources, like books and articles written by scholars using their own sources, then the importance of thinking critically is paramount. You are the scholar now, and that means having a sense, based upon your research, of the validity of another individual's arguments or research. A good example are the books by journalists like Bob Woodward and Ron Suskind. These two individuals produce books based overwhelmingly on interviews with anonymous sources. Because they are both high-profile journalists with access to high-level policy-makers, their books are considered largely credible. Because they do not cite their most important sources, however (and it is important to note that the sources insist on anonymity as a condition of speaking with the journalist), then the reader must beware of the motivations of the sources and the integrity of the journalists. The sources may be using the interview as an opportunity to "get back" at a current or former colleague with whom they had serious disagreements. That desire to use the interview as revenge can degrade the accuracy or eliminate the context of the information being conveyed.
Similarly, the use of memoirs is standard and absolutely accepted. Memoirs are, however, by their nature self-serving recollections. Consequently, claims made by the authors need to be compared against the recollections or records of others who witnessed the same event as that being discussed in the memoir.
In preparing a research paper or presentation, you need to be prepared to be challenged on the accuracy of your information or on your interpretation of events, data, or documents used in the course of your research. That means be prepared to explain the sources of your information and to defend your conclusions. Only by thinking critically when conducting your research can you be prepared for that setting.