In its current use, 'opinion' is generally value-neutral. It does not mean 'mere opinion'. Opinions include responsibly formed beliefs and can even be instances of knowledge. The Oxford English Dictionary's definition reads as follows:
a. What or how one thinks about something; judgement or belief. Esp. in 'in my opinion': according to my thinking; as it seems to me. a matter of opinion: a matter about which each may have his or her own opinion; a disputable point.In an older use, to label something as an opinion implied an epistemological limitation. Opinions were less certain than knowledge. It doesn't follow that opinions are devoid of epistemological value or that they must be irresponsibly formed.
The shift from the older to the current sense is reflected in Webster's Dictionary's changing definitions. In 1828, Webster's says that an opinion is supported by evidence that boosts the probability of a proposition's truth but that falls short of yielding 'absolute knowledge or certainty'. By 1913, Webster's definition hasn't changed much, though it adds that an opinion is a 'belief stronger than impression, less strong than positive knowledge'. Today, Webster's has dropped any suggestion of an epistemological ranking, saying only (in its primary definition) that an opinion is 'a belief, judgment, or way of thinking about something'.
In linking 'opinion' to probability, the older Webster's definitions echo a longstanding tradition in medieval and early modern philosophy. The tradition derives from Aristotle's work. Like Plato, Aristotle regarded opinion (doxa) as inferior to knowledge (episteme). Unlike Plato, Aristotle accorded to opinion a necessary role in our lives and formulated rules for forming rational opinions. He stressed that opinions, while not susceptible of certainty, can be more or less probable. The close link between opinion and probability in Aristotle's works is clearer in ancient Greek, where Aristotle's word for opinion is doxa and one of his words for probability is endoxa. However, what Aristotle had in mind when he wrote of probability is very different from our modern conception. Some of the differences are brought out in Ian Hacking's The Emergence of Probability, the third chapter of which is called 'Opinion'.
For Aristotle, knowledge and opinion have different subject matters. As Sir David Ross put it (in his book Aristotle ), 'Knowledge is of the necessary, opinion of the contingent' (p. 48). Roughly, logic concerns itself with what can be known by demonstrative proof; by contrast, our views on contingent things admit of probability but not demonstrative certainty and, as a result, fall within the purview of dialectic and rhetoric.
Aristotle's approach was hugely influential in medieval thinking about rhetoric (which treated opinion). Indeed, among medieval authors opinions were sometimes referred to as 'probabilities' (or matters of probabilis). According to the authors of a guide to medieval Latin (Mantello and Rigg),
True science requires certitude (certitudo), but it is also possible to have only an opinion (opinio) about physical matters or, somewhat better, to have arguments that fall short of certitude but that are credible (probabilis). Such arguments could include the evidence of the senses. (F. A. C. Mantello & A. G. Rigg, Medieval Latin: An Introduction and Bibliographical Guide [Catholic University of America Press, 1996], p. 357)On that note, I'll conclude this post with another interesting, old logic textbook. It's by Father Peter Coffey, who taught at St. Patrick's College, Maynooth. Coffey studied at Louvain under Cardinal Mercier. According to Thomas Kelly's brief bio of Coffey, 'He was for his time a radical social reformer, supporting trade unionism and, though qualifiedly, Christian socialism, and the ideas of James Connolly' (Thomas A. F. Kelly, The Encyclopedia of Ireland, ed. Brian Lalor [Dublin: Gill & Macmillan, 2003]). Kelly adds that Coffey encountered 'difficulties with ecclesiastical censorship' (ibid.) for his writings on social matters.
Coffey's Science of Logic (1912) echoes the older definition of 'opinion' (and the older definitions given in Webster's). According to Coffey:
In all such cases, where prudent fear of error is excluded, and where the judgment to which we assent is actually true, we have certitude. When such fear is not wholly excluded by the grounds or reasons we have for our assent, the judgment is described as 'probable': our attitude towards it is called 'opinion':2 the mind here inclines towards one of two contradictory judgments as true, but not so strongly as to exclude a prudent fear that it may be false and the other true. ... Opinion, therefore, is the provisional assent of the mind to one of two contradictory judgments, with more or less fear of error. When the fear is so trifling as to be practically negligible, our assent is commonly described as moral certitude. When, on the other hand, our fear of error in assenting to a judgment is exceedingly great, our assent is called a mere suspicion, rather than an opinion. (Coffey, The Science of Logic, v. 2, pp. 212-13)In the footnote (2), Coffey adds, 'Or, also, "belief," in one of the many meanings of this term'.
Wittgenstein didn't like Coffey's logic book, which he panned in his first publication.