We often speak as though we classify our thoughts into different types called facts, opinions, and beliefs.
How do these statements differ from one another? Some philosophers have argued that knowing must mean true and justified belief. However, no one has ever found a test to prove what's justified or true. For example, we all know that the sun rises in the morning. Once, long ago, some people thought this was due to godlike agents in the sky, and that the sun's trajectory was where Apollo steered his chariot. Today our scientists tell us that the sun doesn't really rise at all, because sunrise is simply what we each experience when the planet Earth's rotation moves us into the sun's unchanging light. This means we all know something that isn't true.
To comprehend what knowing is, we have to guard ourselves against that single-agent fallacy of thinking that the I in I believe is actually a single, stable thing. The truth is that a person's mind holds different views in different realms. Thus, one part of an astronomer's mind can apply the common view of sunrise to down-to-earth affairs, regarding the sun as like a lamp that wakes us up and lights our way. But at the same time, that same astronomer can apply the modern physical view to technical problems in astronomy. We each use many different views, and which we choose to use depends, from one moment to the next, upon the changing balance of power among our agencies.
Then if what we believe is so conditional, what makes us feel that our beliefs are much more definite than that? It is because whenever we commit ourselves to speak or act, we thereby have to force ourselves into clear-cut, action-oriented states of mind in which most of our questions are suppressed. As far as everyday life is concerned, decisiveness is indispensable; otherwise we'd have to act so cautiously that nothing would get done. And here lies much of what we express with words like guess, believe, and know. In the course of making practical decisions (and thereby turning off most agencies), we use such words to summarize our various varieties of certainty.
The notion that only certain of a person's beliefs are genuine plays vital roles in all our moral and legal schemes. Whenever we censure or applaud what other people do, we're taught to be more concerned with what those other people genuinely expected or intended to happen than with what actually happened. This doctrine underlies how we distinguish thoughtlessness and forgetfulness from lies, deceit, and treachery. I do not mean that such distinctions are not important, only that they do not justify the simplistic assumption that, among all the mind's activities, certain special kinds of thoughts are essentially more genuine than others. All such distinctions seem less absolute when every deeper probe into beliefs reveals more ambiguities.