example
At its most flagrant, inconsistency is obvious. If I say, ‘All murder is wrong’ and ‘That particular murder was right’, I am clearly being inconsistent, because the second assertion is clearly contrary to the first. (One might be false, both might be false, but both can’t be true.) On a more general level, it would be a bald contradiction to assert both that ‘all murder is wrong’ and ‘not all murder is wrong’. (One must be true and the other false.)
But sometimes inconsistency is difficult to determine. Apparent inconsistency may actually mask a deeper consistency – and vice versa.
Many people, for example, agree that it is wrong to kill innocent human beings. And many of those same people also agree that abortion is morally acceptable. One argument against abortion is based on the claim that these two beliefs are inconsistent. That is, critics claim that it is inconsistent to hold both that ‘It is wrong to kill innocent human beings’ and that ‘It is permissible to destroy living human embryos and fetuses’.
Defenders of the permissibility of abortion, on the other hand, may retort that properly understood the two claims are not inconsistent. A defender of abortion could, for example, claim that embryos are not human beings in the sense normally understood in the prohibition (e.g. conscious or independently living or already‐born human beings). The defender, in other words, might return a rejoinder to the critic that her objection is based on an equivocation (3.3). Alternatively, a defender of abortion might modify the prohibition itself to make the point more clearly (e.g. by claiming that it’s wrong only to kill innocent human beings that have reached a certain level of development, consciousness, or feeling).
Exceptions to the rule?
But is inconsistency always undesirable? Some people are tempted to say it is not. To support their case, they present examples of statements that intuitively seem perfectly acceptable yet seem to meet the definition of inconsistency. One example might be:
It is raining, and it is not raining.
Of course, the inconsistency might be only apparent. What one may actually be saying is not that it’s raining and not raining, but rather that it’s neither properly raining nor not raining, since there is a third possibility – perhaps that it is drizzling, or intermittently raining – and that this other, fuzzy possibility most accurately describes the current situation (3.1).
What makes the inconsistency only apparent in this example is that the speaker is shifting the sense of the terms being employed. Another way of saying the first sentence, then, is that, ‘In one sense it is raining, but in another sense of the word it is not’. For the clauses composing this sentence to be truly inconsistent, the relevant terms being used must retain precisely the same meaning throughout. But, when you do unearth a genuine logical inconsistency, you’ve accomplished a lot, for it can be very difficult if not impossible to defend the inconsistency without rejecting rationality outright. There are poetic, religious, and philosophical contexts, however, in which this is precisely what people find it proper to do.
Poetic, religious, or philosophical inconsistency?
The Danish existentialist philosopher Søren Kierkegaard (1813–55) maintained that the Christian notion of the incarnation (‘Jesus is God, and Jesus was a man’) is a paradox, a contradiction, an affront to reason, but nevertheless true (7.6). Many Christians simply hold the idea to be a difficult mystery.
That kind of difficulty, however, may extend farther than religious contexts. Atheist existentialist philosopher Albert Camus (1913–60) maintained that there is something fundamentally ‘absurd’ (perhaps inconsistent?) about human existence. Post‐structuralist philosopher Jacques Derrida’s theory of différance raises metaphysical questions about the consistency of reality (6.2). Philosophical fiction and poetry may enlist rhetorical strategies involving inconsistency (7.4). Dialetheists and others have even challenged the idea that consistency is fundamental to logic (3.10). Perhaps, then, Emerson was right, and there are contexts in which inconsistency and absurdity paradoxically make sense.
Consistency ≠ truth
Be this as it may, inconsistency in philosophy is generally a serious vice. Does it follow from this that consistency is philosophy’s highest virtue? Not quite. Consistency is only a minimal condition of acceptability for a philosophical position. Since it’s often the case that one can hold a consistent theory that is inconsistent with another, equally consistent theory, the internal consistency of any particular theory is no guarantee of its truth. Indeed, as French philosopher‐physicist Pierre Maurice Marie Duhem (1861–1916) and the American philosopher Willard Van Orman Quine (1908–2000) have separately maintained, it may be possible to develop two or more theories that are (1) internally consistent, yet (2) inconsistent with each other, and also (3) perfectly consistent with all the data we can possibly muster to determine the truth or falsehood of the theories (7.11).
Take as an example the so‐called problem of evil. How do we solve the puzzle that God is supposed to be good but that there is also awful suffering (an apparent evil) in the world? As it turns out, you can advance a number of theories that may solve the puzzle but remain inconsistent with one another. You can hold, for instance, that God does not exist. Or you can hold that God allows suffering for a greater good. Although each solution may be perfectly consistent with itself, they can’t both be right, as they are inconsistent with each other. One theory asserts God’s existence, and the other denies it. Establishing the consistency of a position, therefore, may advance and clarify philosophical thought, but it probably won’t settle the issue at hand. We often need to appeal to more than consistency if we are to decide between competing positions. How we do this is a complex and controversial subject of its own.
SEE ALSO
1 1.12 Tautologies, self‐contradictions, and the law of non‐contradiction
2 2.1 Abduction
3 3.10 Contradiction/contrariety
4 7.2 Gödel and incompleteness
5 7.6 Paradoxes
READING
David Hilbert (1899). Grundlagen der Geometrie
* P.F. Strawson (1952/2011). Introduction to Logical Theory
* Fred R. Berger (1977). Studying Deductive Logic
* Julian Baggini and J. Stangroom (2006). Do You Think What You Think You Think?
* Aladdin M. Yaqub (2013). Introduction to Logical Theory
1.7 Fallacies
The notion of ‘fallacy’ will be an important instrument to draw from your toolkit, for philosophy often depends upon identifying poor reasoning, and a fallacy is nothing other than an instance of poor reasoning – a faulty inference. Since every invalid argument involves a faulty inference, a great deal of what one needs to know about fallacies has already been covered in the entry on invalidity (1.5). But while all invalid arguments are fallacious, not all fallacies involve invalid arguments. Invalid arguments are faulty because of flaws in their form or structure. Sometimes, however, reasoning goes awry for reasons not of form but of content.
When the fault lies in the form or structure of the argument, the fallacious inference
