I recently had a conversation with my mother where she was bemoaning the current bias in media. She is very conservative, so hearing about the liberal media bias is something I've heard about from her for over a decade. But this time her point was different. For the first time in my memory, she called the conservative media sources untrustworthy as well. And if you can't trust Fox News, how do you find out what's really true? Everyone's message is tampered with their own personal biases.
I explained to her how facts ("truth") can and must be extracted from the biased information we receive from others. Nothing can be accepted at face value, even one's own first impressions. In other words, I believe there's no such thing as unbiased information. In contrast, my mother's contention is that people can and do recognize absolute truth, but immoral people twist it to suit their own purposes. I spent a good amount of time thinking about this dichotomy, and I believe what side you fall on depends on how you answer the following two questions:
1) Can absolute truth be perfectly understood?
2) Can absolute truth be communicated without distorting it?
My mother believes that the answer to both of these questions is yes for the vast majority of truths. My personal belief is that absolute truth cannot be perfectly understood, but you can get a really close approximation most of the time. And it's possible but extremely hard to communicate absolute truth without distortion. That's why learning to extract truth from imperfect information is such a crucial skill for a free thinker.
The problem is that statements must fit within the confines of language to be expressed to others, but not everyone has the same underlying definitions. Much of language depends on context. For example, consider this claim by Alcoholics Anonymous:
Alcoholism is a disease.
The majority of lay people believe this is true, while most medical professionals disagree. Whether you agree with this statement depends on how you define a disease. And that in turn depends on what conclusions you want to apply. The impression I get from Alcoholics Anonymous is that it's important to classify alcoholics as diseased so they can receive sympathy for their troubles in life. If alcoholism were caused by personal choices, then friends and family would be more inclined to chastise the alcoholic than rally around and support them in their addiction.
Meanwhile, medical professionals think about diseases differently. When someone has a disease, that means specific kinds of medication and treatment may be effective. From a medical perspective, diseases tend not to require psychotherapy to treat. So calling alcoholism a disease is misleading for the purposes of treatment. But it's a reasonable statement if you define a disease as, "a malady that requires outside assistance to treat."
The question is which definition of disease are people thinking of when you use the word, and I tend to think the medical professional definition is the more common usage. After all, if all ailments that required psychiatric help were considered diseases, that would include depression and schizophrenia, and no one is claiming that is the case. That's a sign the statement "alcoholism is a disease" lacks internal logical consistency. Nevertheless, that statement is still true within the context of a rather contrived set of definitions.
Perhaps its best to think about statements as belonging to one of two categories: scientific or sociological. A scientific statement is any statement that uses precise terms that are universally understood and agreed on. For example, "the sum of 0 and 1 is 1." Scientific statements, being the basis of science, must be both well defined and measurable.
A sociological statement is anything that's not scientific and therefore not well defined. For example, "Pablo Picasso was a great artist." Not all sociological statements are obviously opinions, however. The statement, "Rappers are musicians" depends on your definition of musician. I personally think of rap stars as poetry artists and not musicians, because their work lacks both melody and harmony.
Even statements that are thought of as facts can be disputed. Consider the following statement:
Senator Obama won a decisive victory over McCain in the past presidential election.
Most but not all people believe this statement is true. The disagreement centers around the definition of decisive. It's true that Obama had over double the electoral votes of McCain, but Obama only had 13% more votes than McCain had. Some people claim that a victory is not decisive when 46% of the country voted for the loser. (I disagree for a variety of reasons, but that's not really the matter at hand.)
The further statements get from scientific statements things like "0 + 1 = 1" and the closer they get to sociological statements like "Rappers are musicians", the harder they are to agree upon and therefore test. And as a result, their absolute truth becomes impossible to verify and bias works its way in. Unfortunately, it is sociological statements that interest most of humanity, not scientific ones. Almost every statement you've read in this post has been a sociological one-- its truth can only be approximated, not scientifically tested and verified. The vast majority of "facts" you interact with on a daily basis are untestable. That is why its so important to analyze whatever you hear rather than accepting it as fact. It's also why artificial intelligence is the most difficult area of computer science. AI is the area that deals with answering sociological questions like "what is best" or "what do you want".