Reasoning by deduction

Reasoning by deduction (deductive reasoning) is the form of reasoning which begins with one or more propositions (a premise or premises) from which a conclusion is reached by following a series of reasoned steps.

Assuming you have some basic background in mathematics, you are already familiar with deductive reasoning. For example, if A = B and A = 5, it follows that B = 5. It also follows that both 2A and 2B = 10 and that A – B = 0; and so on, through as much juggling with the value of A as you wish to pursue.

The form of deductive reasoning which lawyers use is called a syllogism and is best explained and illustrated by an example.

If A = B

And if B = C

Then A = C.

The first line of a syllogism is called the major premise, the second line is called the minor premise, and the third line is called the conclusion. Putting this into a specifically legal context, the major premise is a proposition of law; the minor premise is a proposition of fact; and the conclusion is the decision in the case. For example:

it is an offence to exceed the speed limit in a motor vehicle on a public highway (major premise/proposition of law);

exceeding the speed limit in a motor vehicle on a public highway is what the defendant did (minor premise/proposition of fact);

therefore

it is an offence to do what the defendant did – or, in other words, the defendant is guilty (conclusion/decision).

In reality, of course, simply being able to identify the form of reasoning as syllogistic tells us very little. What we really need to know is how we can identify and formulate:

the correct proposition of law (the major premise); and

the facts to which the proposition of law must be applied (the minor premise).

The answers to these questions are, respectively:

by identifying the relevant sources of law and applying the techniques of legal method to those sources; and

by evidence establishing any facts which are in dispute.

As it is only the first question which falls within the scope of this chapter, it is only that question which we shall consider. However, before doing so we must consider the question of defective deduction. The judgment of the Court of Appeal in Ward v James [1965] 1 All ER 563 provides a suitable example.

The case arose at a time when it was still common for juries to take part in trials where at least one issue was the amount of money which should be awarded by way of damages for personal injuries. However, there was a growing recognition that juries were not very good at assessing the cash value of pain and suffering and, as a result, were awarding wildly differing damages in very similar cases. (The fact that this was seen as being unjust is an illustration of the point – which is also made at p. 104 – that one element of justice is that similar cases should be treated in similar ways.) In Ward v James, a unanimous Court of Appeal, led by Lord Denning MR, came to two conclusions. First, juries should not normally hear personal injury cases. Secondly, where there are special circumstances which justify the participation of a jury in such a case, the jury’s role should be restricted to determining the facts, rather than extending to the assessment of damages.

However, if we remind ourselves of the premise from which the argument starts (juries are not very good at assessing damages in personal injuries cases), the only conclusion we can validly draw is that juries should not assess damages in such cases. In other words, the court’s conclusion that juries should normally have no role at all is simply not supported by valid reasoning.

For example, in a typical case involving a claim for damages in respect of personal injuries arising from a road traffic accident, a jury composed of ordinary motorists may be particularly well-equipped to decide whether the quality of the defendant’s driving was up to the standard required of the reasonably competent driver. Admittedly, if the defendant is found liable it may well be better for damages to be assessed by the judge rather than by the jury, because the judge will know what sort of damages are being awarded by other judges, which will result in much greater consistency between cases. But this is a long way from excluding juries altogether. (Also, if only by way of an aside, it is worth mentioning that adopting this solution to the problem of juries’ inability to assess damages sensibly would reflect the position in criminal trials in the Crown Court. In that context, the jury decides whether the evidence has proved the facts of the case beyond reasonable doubt; but, if it does come to that conclusion, the judge undertakes the final stage of proceedings, namely deciding on the appropriate sentence.) In short, therefore, both of the court’s conclusions in Ward v James may appear, at first glance, to be the result of valid reasoning, but more careful consideration shows that only the second one was.

The fact that the decision on the first point resulted from defective reasoning did not prevent this case from becoming an important factor in the decline of the use of juries in civil cases. This may seem odd, but perhaps the obvious correctness of the decision itself had something to do with this. The important point of principle is that defective reasoning does not become valid simply because the conclusion it claims to establish happens to be correct.

We will now turn to the importance of being sensitive to language and the importance of context.