To claim absolute certainty about the truth of a proposition may be to claim the right to ignore or be blind to further evidence affirming or denying the truth of that proposition.
Logically, if you say that you're certain about something, then you should be prepared to wager everything on it. If you're not prepared to wager everything on it, then you must not really be certain about it (unless there are psychological, social, or other factors that prevent you from making such a wager, despite your alleged certainty).
Of course, if you say that it's not in your nature to wage everything on something, then your reluctance to wager everything (or to wager anything at all) may be due to your recognizing that you can never be absolutely sure of, or certain about, anything.
Benjamin Franklin's well-known saying (now so often quoted that it has become a cliché) regarding certainty appeared in a letter to the French scientist Jean-Baptiste Leroy, written on November 13, 1789: "In this world nothing can be said to be certain, except death and taxes."1
Other certainties may include mathematical certainties (such as "2+2=4"), physical certainties (such as "marble is harder than limestone"), cosmological certainties (such as "Ganymede is the largest moon of Jupiter"), and historical certainties (such as "The Battle of Bosworth Field occurred in 1485").
F.W.J. Schelling (1800) says that "for certainty in theory we lose it in practice, and for certainty in practice we lose it in theory" ("über der theoretischen Gewissheit geht uns die praktische, über der praktischen die theoretische verloren").2
I can't be more certain of anything than I am of the fact that some (though perhaps not all) lost opportunities are lost permanently. Perhaps this is merely a certainty of my own (and more generally of human) finitude. I may only be certain of my own fallibility and uncertainty.
Believable fiction may depend on fictionalization of the real and realization of the fictional.
If to be thoughtful is to be careful, and to be thoughtless is to be careless, then you might suppose that to think is to care. But unfortunately that's not the case.
It may always be possible to say "I don't know" with greater certainty than "I know."
If I'm feeling pain, and it's a pleasurable pain, then I may be uncertain whether I'm actually feeling pain.
Epistemic duty (or the ethics of knowledge) may require that we have adequate grounds for saying we're certain about something. It may also require that we have adequate grounds for saying we're doubtful about something.
Wittgenstein asks, "Doesn't one need grounds for doubt?"3 But it can be argued that we aren't required to believe a proposition to be true just because we can't prove it to be false.
If we qualify our statements about certainty by saying, "We think (but aren't sure) we're certain about that," then we may be saying we're relatively certain, but not absolutely certain. Relative certainty may be susceptible to doubt, while absolute certainty may be immune to doubt. The former may be much easier to attain than the latter.
If we tell others that we know something, then we may have a duty to let them know when we're not absolutely certain that we know. Our claim to know something requires some degree of certainty on our part. We don't always have to be absolutely certain about something in order to properly claim to know it. Relative certainty may be sufficient in some cases. But we must at least have met some threshold level of certainty about something in order to properly claim to know it.
The term "objective certainty" may be ambiguous, insofar as it may refer to the objectivity of the fact that certainty has been attained or the objectivity of the reasons for which certainty has been attained. In the first sense, certainty can objectively be said to have been attained, while in the second sense, certainty can be said to have been attained objectively. It may be important to distinguish between the two senses. For example, Bill can objectively be said to be certain about something without his having objective reasons for being certain about that thing.
Subjective certainty may include psychological and intuitive certainty. Objective certainty may include epistemic, propositional, and evidential certainty.
One way of expressing subjective certainty may be to say, "I'm certain, but I could be wrong." This way of hedging one's bets about certainty may be a way of immunizing oneself from error—or it may just be a way of equivocating.
Subjective certainty doesn't entail objective certainty. We may be subjectively certain about something, but objectively wrong about it.
There may be various kinds of certainty (psychological, moral, epistemic, deductive, and statistical). There may also be various degrees of certainty (which may be expressed by such adjectives as "absolute," "high," "low," or "relative.")
However, Peter Unger (1975) argues that there are no degrees of certainty, and that "certainty" is an absolute term, like "flat." Just as nothing (or hardly anything) in the world is (absolutely) flat, nothing (or hardly anything) in the world is (absolutely) certain. Unger thus argues for scepticism regarding the possibility of knowledge, by saying that since knowledge entails (absolute) certainty, nothing (or hardly anything) is known.4,5
Peter Klein (1981) also argues that knowledge entails absolute (psychological and evidential) certainty, and that if S knows that p on the basis of some evidence e, then e renders p absolutely certain for S. Klein argues that S knows that p only if p is absolutely certain for S, but he makes clear that he's not saying that the ordinary understanding of the meaning of the term "knowledge" restricts the term to only those cases in which a proposition p is absolutely certain.6 There may be relative as well as absolute uses of the term "certainty."
Harry Frankfurt (1962) says that there may indeed be degrees of certainty, and that some things may be regarded as more certain than others. We may be willing to wager more on the truth of one proposition than we're willing to wager on the truth of another proposition, if we regard the one's truth as more certain than the other's.7
Jason Stanley (2008) says that knowledge doesn't entail certainty, and as an example he provides the statement, "I know that Bill came to the party. In fact, I'm certain that he did," which seems to show that we can know something without being certain of it. As another example, he provides the statement, "John knows that Bush is a Republican, though, being a cautious fellow, he's only somewhat certain of it," which seems to show that we can know something without being completely certain of it. Stanley explains that there's nothing strange or odd about ascribing knowledge of something to someone who's not completely confident that she does indeed know that thing. We don't automatically object when someone is described as knowing something of which she isn't certain.8
Stanley also explains that fallibilism in epistemology (the theory that knowledge is compatible with a lack of certainty) is compatible with both "the knowledge norm for assertion" (that we should assert p only if we know that p) and "the certainty norm for assertion" (that we should assert p only if we're certain that p). Thus, while fallibilism holds that we can know p without being certain that p, it doesn't hold that we can say we know that p if we don't know or aren't certain that p.9
Gaining greater knowledge of something may sometimes lead us to feel less certain about that thing. By learning more about that thing, we may sometimes develop a greater appreciation of the limits of our knowledge.
Second-order certainty may be exemplified by such statements as, "I feel certain that I'm certain."
The certainty of faith may be an example of psychological, moral, theological, or doctrinal certainty. It may be based on belief in truths that cannot be proved by reason or that surpass our understanding. It may be based not on empirical proof or scientific testing, but on the power of authority, scripture, testimony, or revelation.
1Benjamin Franklin, The Works of Benjamin Franklin, Vol. XII, Letters and Miscellaneous Writings, 1788-1790.
2F.W. J. Schelling, System of Transcendental Idealism, translated by Peter Heath (Charlottesville: University Press of Virginia,1978), p. 11.
3Ludwig Wittgenstein, On Certainty, edited by G.E.M. Anscombe and G.H. von Wright, translated by Denis Paul and G.E.M. Anscombe (Malden: Blackwell Publishing, 1969), p. 18c.
4Peter Klein, Certainty: A Refutation of Scepticism (Minneapolis: University of Minnesota Press, 1981), p. 131.
5Peter Unger, Ignorance: A Case for Scepticism (Oxford: Oxford University Press, 1975), p. 49.
6Klein, Certainty: A Refutation of Scepticism, p. 117.
7Harry G. Frankfurt, "Philosophical Certainty," in The Philosophical Review, Vol. 71, No. 3 (July 1962), pp. 303-327.
8Jason Stanley, "Knowledge and Certainty," in Philosophical Issues, Vol. 18, Issue I (September 2008), pp. 41-42.
9Ibid., p. 56.