An argument is logically valid if and only if it follows an argument scheme that is logically valid. An argument scheme is logically valid if and only if it is impossible for any argument following this scheme to have true premises and a false conclusion. See, for example, the logically valid argument scheme that is called modus ponens:
In this modus ponens argument the conditional “if p, then q” is understood as being true under all circumstances: whenever the statement “p” is true, the statement “q” is also true. That means, the statement “if p, then q” is understood here as something like a law of nature that explains an event described by q as a necessary consequence of an event described by p. If it happens, by contrast, that “p” is true but “q” false, then—and only in this case—the connected statement “if p, then q” is false. So, if the conditional “if p, then q” and the statement “p” in the modus ponens argument above are supposed to be true under all circumstances, it should be impossible to find any two propositions p and q (that is, complete and well-formed sentences that can be true or false) that would lead to a false conclusion. Take the following example:
It is obvious that the conclusion of this argument must be true if both the premises are true. Since it is impossible to imagine that this would be different for any argument that is structured according to this modus ponens form, we can conclude that the form itself is logically valid and, thus, any argument that follows this form.
What logical validity means in the AGORA approach
Please note the following important points that are specific for the AGORA approach:
1 In the literature a distinction between “strict modus ponens” and “defeasible modus ponens” has been proposed. In the defeasible form the law-like premise “if p, then q” is often modified by a qualifier such as “usually” or “as a rule,” or there is an additional premise such as “It is not the case that there is an exception to the rule that if P, then Q” (Walton, Reed, & Macagno, 2008, p. 366). However, the same effect can be achieved by establishing the convention that every statement is only “believed to be true” by someone whose name is connected to this statement. This convention can be justified within a pragmatist framework according to which all empirical knowledge is fallible. If knowledge is fallible (i.e., it can be false), the focus shifts to improving our beliefs in the long run or to justify what we believe we know. Whether a statement used in an argument is indeed true is thus less important than the question whether we can provide a justification for it, and whether our justification gets accepted. As long as nobody cares to defeat the reasons for our claims, they can be accepted as true. This, for us, is the essence of a dialogical and pragmatist approach to argumentation (see also Pinto, 2001).
2 Any statement that is justified by a logically valid argument cannot itself be attacked within an AGORA map. The reason for this design decision is that such a statement is necessarily true if all the premises are true, so that the attention of an opponent should be directed to these premises. However, since any statement—even though necessarily true based on a certain argument—can be “framed” from a different point of view, it is possible to attack such a statement in another argument map.
Pinto, R. C. (2001). Argument, Inference and Dialectic. Collected Papers on Informal Logic. Dordrecht; Boston: Kluwer Academic.
Walton, D. N., Reed, C., & Macagno, F. (2008). Argumentation schemes. Cambridge ; New York: Cambridge University Press.