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Sources who demonstrate objectivity have no bias—prejudice or partisanship—that would prevent them from making an impartial judgment on your speech’s topic. People can be biased for several reasons. Some have economic self-interest, or the desire to make money, so they may slant facts or explanations to make certain alternatives seem more attractive. Others may need to please superiors—for example, a government worker who defends poorly conceived government policies. Still others have what’s called ego investment: they’re so wrapped up in a pet theory or cause that they lose their ability to evaluate it with an open mind.
Needless to say, you should avoid evidence from biased sources. If you use it, audience members will be unlikely to accept the point you are trying to prove.7 Furthermore, you can’t be confident that you have met your ethical duty to present truthful facts to the audience.