You may be wondering why anyone would want to talk about "balancing" objectivity and persuasion when academic writing is so largely a matter of documentation, rigorous testing, and other objective controls. Academic writers know they cannot merely assert something as true - express a personal opinion - without acknowledged factual support. Research and objectivity are essential, we assert, to formal academic writing.

All true. And yet, no matter how "objective" your facts, statistics, results of experiments, or quotations from respected sources, your argument is still by its very nature "subjective." It is, after all, your hypothesis, your experiment, your vision that you wish your readers to take seriously. You inevitably want your readers at the very least to say, "your idea merits consideration." To get this result, you must be persuasive, even as you base your position on objective evidence.

Ancient Greek students of argument assigned names to three forms of rhetorical appeals to ask for the audience's assent. In Greek, these appeals are identified as ethos, pathos, and logos, or ethical, emotional, and logical appeal. All three are tools which you can use to try to make an argument acceptable to an audience.

Ethos or ethical appeal: focuses on the writer (or speaker), presenting him or her as a person worthy of the reader's trust. You are trying, in other words, to create an appropriate image for yourself in the text. Much of your success will depend upon the audience to whom you are appealing, although it's safe to say that for academic degree purposes, your audience is your advisor and your committee. But in the course of your career, your audiences will vary. Ethos is fundamentally a writer's attempt to adjust to the tastes of the audience in question. In writing, you must depend upon your choice of words and their arrangement, as well as tone, to convey your ethos.

Pathos or emotional appeal: aimed to affect the specific inclinations of the audience. This will probably be your least used appeal in your graduate school writing, if indeed you'll use it at all. Outside of graduate school, a pathos appeal can be highly effective - or a total disaster. Use sparingly and handle with care.

Logos or logical appeal: derives from the intellectual understanding shared by the arguer and the audience. This is clearly your most valuable tool for academic writing, and is the direct link between objectivity and persuasion. If you can persuade your audience to acknowledge the logos of your argument - which includes all facts, statistics, definitions, analogies, quotations from authorities, and other evidence offered in support of your claims - then you have truly learned to balance objectivity and persuasion.
(thanks to Bradbury & Quinn)

In summary, "the most successful arguments rest on a firm foundation of solid, logical support. In addition, many arguments include emotion because it can play an important part in swaying reader opinion. Furthermore, writers often make ethical appeals by projecting favorable images of themselves since readers form conclusions based on their judgement of the writer."
(thanks to J. Ferganchick-Neufang)