In this (very) short article, I am going to try to explain what makes an argument valid (comparing two views), what makes an argument sound (again comparing two corresponding views) and then I aim to distinguish ‘good’ arguments from either of these. I will attempt to explicate why validity, on either interpretation, will be a necessary but insufficient condition for soundness (on that respective interpretation). It will turn out that soundness (on either interpretation) will not be a sufficient or a necessary condition for goodness, and that validity (on either interpretation) will be a necessary but insufficient condition of goodness. It will also be shown that goodness is neither a necessary nor sufficient condition of soundness. This article can, perhaps, serve as a useful prolegomenon to introductory deductive logic, though its distinctions are themselves somewhat unorthodox and reach beyond the scope of formal logic.
One definition of validity which is relatively common, easily found in most introductory textbooks on deductive logic, is the following:
An argument is valid if and only if it is not logically possible for the premises to be true and the conclusion false.
Let’s explore the dynamics of this definition. It would mean that an argument of the following sort would be considered valid:
- All men are human
- Socrates is a man
- Therefore, Socrates is human.
Clearly, in this argument, it is not logically possible for the premises to (both) be true, and for the conclusion to be false. The same can be said of the following argument:
- Bob loves Carroll
- If Bob loves Carroll then Carroll loves Joe
- Therefore, Carroll loves Joe
This is pretty obviously logically valid. So too, though, is the following argument:
- All women are purple.
- Socrates is a woman.
- Therefore, Socrates is purple.
The reason this argument is valid is that it is not possible for the premises to (both) be true, and for the conclusion to be false. Perhaps the premises and conclusion are all, in fact, false, but in any possible world in which the premises were true, the conclusion would be true. Thus, validity is not concerned with truth so much as truth-preservation. The concern is to ensure that one cannot, in a ‘valid’ argument, move from true premises to a false conclusion. Take the following example as well:
- All women are purple.
- If all women are purple, then evolution is true.
- Therefore, evolution is true.
In this argument, we have a conclusion which is (I presume) true in fact, while the premises are all false. However, the argument is clearly valid as well, since it is not logically possible that the premises be true and the conclusion false. Remember that validity requires nothing more than that it is not possible for both (i) the premises to be true, and (ii) the conclusion to be false.
The difficulty with this account of validity arises when we are confronted with examples of the following variety:
- All men are animals.
- If all men are animals then Tyrannosaurus Rex makes a good pet.
- Therefore, 3+4=7
This argument is logically valid, since it is not logically possible for the premises to be true while the conclusion is false (mostly because it isn’t possible for the conclusion to be false, and it is ‘possible’ for the premises to be true). Such an argument, however, doesn’t have any dialectical appeal. Consider also:
- I once drew a square-circle,
- If I once drew a square-circle, then I am a married bachelor,
- Therefore, I once drew the impossible.
This argument can be tricky; in order to find out whether it is valid we have to ask whether it is possible for both (i) the premises to be true, and (ii) the conclusion false. As it turns out, it is not possible for the premises to be true and the conclusion false, precisely because it is not possible for the premises to be true. Thus, formally speaking, it is a logically valid argument.
The definition of a sound argument is pretty straightforward: an argument is sound if and only if it is (i) logically valid, and (ii) all of its premises are true. For example,
- Socrates was mortal.
- Everything that was mortal, was once alive.
- Therefore, Socrates was once alive.
In this argument, we find that it is not logically possible for the premises to be true while the conclusion is false, and in addition, we find that both premises are clearly true. Thus, we have a sound argument on our hands. Any argument which is logically valid is sound just in case all of its premises are true. Thus, for example, the following argument is sound:
- A tautology is a tautology.
- The sentence ‘is this a question‘ expresses a question.
This exemplifies the problem with the formal definitions of validity and soundness. It shows that one can construct sound and vacuous arguments by simply ensuring that the premises and conclusions are all necessary truths, or at least that the conclusions are necessary truths while the premises are true. In the interest of more off-the-cuff examples, take for instance:
- I once wrote this sentence.
- If I once wrote this sentence, then I have written at least one sentence.
- Therefore, 3+4=7
This argument is both logically valid, and sound, and yet it appears to be a very bad argument. Nobody who didn’t already accept the conclusion could be led by it to accept the conclusion. It is a bad argument, even for those of us who accept the conclusion; if this argument were submitted as our reason for believing the conclusion then our mathematical belief that 3+4=7 would literally be unjustified (a necessary self-evident truth in which we believe can, of course, be unjustified). What all this illustrates is, first, that the formal definitions of validity and soundness are concerned only with truth preservation, and not with the persuasive force of an argument at all. As philosophers who specialize in the study of modal logic often make a distinction between ‘broad’ and ‘narrow’ logical possibility (eg. a square-circle is broadly logically impossible, but narrowly logically possible since there isn’t any purely formal way to evidence a contradiction between the predicates ‘square’ and ‘circle’ – or, at least, there may not be, depending on how ‘narrow’ we’re being), so too, perhaps, should we make a distinction between broad and narrow validity & soundness. What we have looked at so far would be the purely formal or ‘narrow’ accounts of validity and soundness. Maybe a ‘broad’ view of validity (which I will henceforth write as ‘validity*’) would be something like: an argument is valid* if and only if i) it is not possible for the premises to be true while the conclusion is false, and ii) the conclusion meaningfully follows from the truth of the premises. This definition of validity* says everything the former one did, with the addition that the premises and conclusion have to be semantically related (i.e., meaningfully related; they have to have something to do with one another). We can correspondingly say that an argument is sound* just in case it is valid* and its premises are true.
Now, validity* and soundness* are not appropriate distinctions in an introductory course on deductive logic, and so are somewhat philosophically unorthodox. However, they are rather useful outside of that narrow context, and in the context of doing philosophy. In philosophy, we don’t just want sound arguments, we want sound* arguments!
Speaking of what philosophers want, there is another issue I wish to examine, which is what makes an argument ‘good’ by philosophical standards. It turns out, I will argue, that neither soundness nor soundness* are necessary or sufficient conditions of ‘goodness’.
I submit that the goodness of an argument consists in two things: i) that the argument is logically valid*, ii) that the accumulated uncertainty of the premises to the argument’s intended audience sets a reasonably high lower bound on the probability of the conclusion. This second criterion is specially crafted to avoid the common mistakes which have, in the past, been made even by some relatively good philosophers like William Lane Craig; namely, the mistake of thinking that premises in a valid argument need be merely each more plausible than their respective negations for the conclusion to follow forcefully. Indeed, the (probability of the) premises of an argument merely set a lower bound on the probability of the conclusion. If that lower bound on the probability of the conclusion is less than or equal to 0.5 then the argument is not compelling. Whether an argument is persuasive or not to some subject is going to depend on their appraisal of the premises, of course, but a good argument will consist of premises which are not merely more plausible than not, but also highly plausible – plausible enough, at least, that the conclusion will also seem highly plausible. This definition obviously subjectivizes ‘goodness,’ making it dependent upon an audience’s appraisal, but that shouldn’t bother us very much because plausibility has to figure into the goodness of an argument in some way, and ‘plausibility’ is already a term of epistemic appraisal.
Consider the following two arguments, both of which are valid and at least one of which is sound. First, the modal ontological argument, which we can roughly reconstruct as:
- God possibly exists (i.e., God exists in at least one logically possible world).
- If God exists in one logically possible world then God exists in all logically possible worlds.
- If God exists in all logically possible worlds then God exists.
- Therefore, God exists.
This argument is sound just in case the conclusion is true. However, that doesn’t make it a very good argument in my sense. Indeed, consider its parody:
- God possibly does not exist (i.e., there is at least one logically possible world in which God does not exist).
- If there is at least one logically possible world in which God does not exist, then there is no logically possible world in which God exists.
- If God exists in no logically possible worlds then God does not exist.
- Therefore, God does not exist.
At least one of these two arguments is valid, valid*, sound and sound*, but it is arguable that neither of them are good. Goodness, then, consists in more than just soundness*. So, given the way I’ve just outlined things, we can imagine any number of arguments which are good without being sound, sound without being good, valid* without being sound, sound* without being good, but none which are good without being valid*. The goodness of an argument, it seems, is largely in the eye of the beholder; the goodness of a valid* argument is entirely in the eye of the beholder.