It can even happen that an argument is rationally persuasive for you at one time, but not at a later time, because you have acquired new evidence which defeats the argument (by suggesting that the conclusion is false, more strongly than the argument suggests its truth). To be strictly accurate, then, we should speak of rational persuasiveness for a person at a particular time. For the sake of simplicity, however, we have omitted this complication from what is already a rather complex definition.
There are six further points to bear in mind as regards rational persuasiveness.
1 It is not possible for the conclusion of a deductively valid argument to be defeated by a person's total evidence - this is only possible for inductively forceful arguments. Thus condition (iii) of the definition of rational persuasiveness only applies to inductively forceful arguments. The reasons for this are:
a If you accept with good reason the premises of an argument which you recognise to be deductively valid, then you must accept the conclusion as well; for you know that if the premises are true, then the conclusion must necessarily be true also. If a person has good reason to accept the premises of a deductively valid argument, then we may conclude that it is rationally persuasive for that person without further ado. Any further consideration which, for that person, casts doubt on the conclusion, must equally cast doubt on the premises.
Why is this so? Well, the answer is because it is part of the definition of a valid argument that if its premises were true, then, necessarily, its conclusion is true. Remember from Chapter 2: validity has nothing to do with the actual truth of the premises - so even if you don't have reason(s) to accept one or more of the premises, the argument remains valid (this is just to say that valid arguments can have false premises). But if the premises are true and the argument is valid - the conclusion must necessarily (no exceptions) be true. So if you have reason to accept the premises then those very same reasons are equally strong reasons to accept the conclusion. The only way that a deductively valid argument can fail to be rationally persuasive is if a person is without reason to accept the truth of one or more of the premises (remember that since the argument is deductively valid, condition (i) has already been fulfilled), b The adverb 'probably' (or similar term) before the conclusion of an inductively forceful argument allows that it is possible that the premises are true and the conclusion false. When you have an argument that is inductively forceful, the third condition of the definition of rational persuasiveness becomes relevant; for the evidence that defeats the conclusion is precisely the evidence that shifts the balance of probability. Because inductively forceful arguments only claim that if the premises are true the conclusion is likely to be true, there is room for situations in which the conclusion is actually false.
Condition three of the definition of rational persuasiveness means that there are two ways in which an inductively forceful argument can fail to be rationally persuasive. The first is when you have no good reason to accept one or more of the premises (i.e. it fails condition (ii)). The second is when the argument is defeated for you: you have evidence that means you can't accept the conclusion even though you accept that the premises are true and the argument is inductively forceful.
2 It is not part of the definition that the argument be sound (either deductively or inductively). The reason is simple: the notion of rational persuasiveness is intended to capture what it is about an argument that makes it acceptable to a person. It is the notion of an argument's giving a person good reason to accept its conclusion. Indeed, an argument can have a false premise, and hence be unsound (neither deductively nor inductively sound), yet be rationally persuasive for a person. To illustrate, take the case of the weighted coin again. Suppose that on this one rare occasion, the coin settles to the bottom of the cup heads down, so P2 is false. Still, in that case, Barney would still be quite reasonable in thinking that P2 is true (for he knows that the coin is weighted). So the argument would still be rationally persuasive for Barney, despite its having a false premise, and thereby being unsound. He would be quite right - in the sense of being rationally justified - to accept the conclusion of the argument, even though, in fact, it is false.
This point about rational persuasiveness illustrates an important fact to which we will return later in much more detail: a person may reasonably believe a proposition which is, as it happens, false. In other words, there is such a thing as a reasonable mistake. This is an elementary point, but it is an easy one to forget - and hence, for example, inappropriately to blame people for their mistakes. Indeed, a useful way to put the point is to say that the word 'mistake' is ambiguous. To say that someone is 'mistaken' could mean either (i) that they have accepted a false conclusion, or (ii) that they have been persuaded by bad reasons - by an argument which is not in fact rationally persuasive for them (or have failed to be persuaded by good reasons - by an argument that is rationally persuasive for them). Clearly we are responsible for our mistakes of type (ii); we ought to be persuaded by good reasons, and not by bad reasons. If we fail in this, then, typically, we are blameworthy. But it is much less clear that we are responsible for mistakes of type (i). If someone believes a proposition on the basis of good reasons - on the basis of arguments that are rationally persuasive for him or her - then it might just be bad luck if the proposition turns out to be false. If so, then the person need not be to blame for having made a 'mistake'.
Suppose, for example, that you are a doctor. Suppose there is a drug X which you know to have cured a certain dangerous disease every single time it has been used, in over a million cases; furthermore, you know that it has never had a negative side effect, and contains no substances known to be dangerous in any way. A patient has the disease and you prescribe X. Unfortunately, instead of being cured by X, the patient is made ill by it. Were you mistaken in believing that X would safely cure the disease? Not if by 'mistaken' we mean a mistake of type (i), and hence not in the sense required if you are to be blameworthy. Indeed, you would have been open to criticism if you had not prescribed the drug, since you would have been going against a massive body of evidence.
3 It should be appreciated why we have named rational persuasiveness as we have. Remember that at the beginning, we said that there are various kinds of attempts at persuasion. This book is about trying to distinguish argumentative from non-argumentative - chiefly rhetorical -attempts at persuasion, and learning to evaluate them. An attempt at persuasion by argument is an attempt at rational persuasion, as opposed to other kinds of persuasion, which do not appeal to your reason (but rather to your emotions or prejudices). Directed at you, it is an attempt at providing you with a rationally persuasive argument. Note, again, that this does not mean that the argument must be a sound argument. The attempt depends upon what you reasonably believe regardless of whether or not those reasonable beliefs are in fact true.
4 Rational persuasiveness is a matter of degree; it is not all or nothing. Of two arguments for the same conclusion, one may be more rationally persuasive for you than the other, even if both are rationally persuasive. For example, suppose you have good reason to believe that if the barometric pressure is low, then rain is likely; you also have a slight reason to believe that if the wind is from the west, then rain is likely. Suppose you know that the barometric pressure is low, and that the wind is from the west. So you have two arguments for the same conclusion. But the one based upon the barometric pressure is more inductively forceful. In that case that argument is more rationally persuasive for you than the other.
5 'Rationally persuasive' does not mean merely 'persuasive' or 'convincing'. A rationally persuasive argument may fail to persuade anyone. Whether or not an argument is rationally persuasive for you does not depend upon whether you think it is. The crux of the matter is to understand this: an argument may be rationally persuasive for you even though you are not persuaded by it. This should not be regarded as paradoxical. All it means is that there are cases where you ought to be persuaded by an argument, but you are not. Likewise, there are cases where you are persuaded or convinced by an argument, but where you should not be, because the argument is not actually rationally persuasive for you. It is the task of rhetoric to cause people to overestimate the rational persuasiveness of an argument - to convince or persuade people without actually giving them good reasons.
To understand why this is so, examine Figure 6.1. You can see from this figure that when we examine the relationships between sound arguments, rationally persuasive arguments and the arguments that actually persuade people, it is possible for a given argument to be one, two or all three of them. The important point you need to note is that rational persuasiveness and soundness are properties that arguments can have independently of whether an individual or group actually finds them persuasive. Human beings are often irrational and part of the function of the concept of rational persuasiveness as we have defined it is to acknowledge this. If rational persuasiveness were defined so as to make all rationally persuasive arguments ones that actually persuaded people, we would have no way of accounting for the fact that sometimes people fail to be persuaded by arguments that they really should be persuaded
by, and are sometimes persuaded by arguments that they should not be persuaded by.
There are three ways in which one can be mistaken about the rational persuasiveness of an argument. Study them carefully, especially if the preceding paragraphs are mysterious to you.
First, we can make mistakes concerning whether or not an argument is valid or inductively forceful. Especially in the case of a logically complex argument, we may for example think the argument deductively valid or inductively forceful when it is neither (for example when the argument contains a fallacy which is also an effective rhetorical ploy; see Chapter 4). If so, then even if we accept the truth of the premises, and are perfectly justified or reasonable in doing so, we may think that the argument is rationally persuasive for us when it is not. Equally, if we think an argument invalid when it is valid, or inductively unforceful when it is inductively forceful, it is possible to think an argument rationally imper-suasive for us when in fact it is rationally persuasive for us.
Second, we can be mistaken in thinking we have good reason to accept a premise as true. For example, consider the following argument:
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