The sources of knowledge III - deduction and induction

8.3 Background

So far we have focused on empirical sources of knowledge. But there are also non-empirical sources, namely, the faculty of reason. Our faculty of reason is said to provide us with what is called a priori knowledge. It is thus important to understand the distinction between a priori knowledge and empirical knowledge. The distinction fits into a larger debate over the role of reason in our coming to have knowledge. On the one hand, we have Empiricists, who maintain that knowledge is derived from observation (and the place of reason is only to draw inferences from what is provided by experience). Rationalists, on the other hand, argue that it is possible to gain some knowledge independently of experience, and thus that reason can supply us with new knowledge. It helps in understanding what a priori means to distinguish between what causes a belief and what justifies a belief. The cause of a belief might be empirical, but how we come to justify it, or come to know the truth of it, might not be. So, we could say that for the rationalist, we come to know the truth of an a priori belief without recourse to experience, although experience might have played a role in causing us to think of the belief.

Here is Oxford’s Professor Dan Robinson on the possibility of Kant’s synthetic a priori judgements.

Optional activity: A priori and empirical knowledge

Read Huemer chapter 3 pp. 125-30.

State in your own words the distinction between a priori and empirical knowledge. How does a priori knowledge differ from innate knowledge? What does Kant mean by 'synthetic a priori knowledge'?

You may want to share your thoughts with others in the Deduction and induction forum.

Much of our knowledge of the world is gained via inductive inferences; that is, by making inferences where the truth of the premises does not guarantee the truth of the conclusion. For example, suppose one made the following inference:

Every observed emu has been flightless. So, all emus are flightless.

If one has observed an awful lot of emus, in lots of different environments and at lots of different times, then we would regard such an inference as acceptable. Notice, however, that even if the premise (the first sentence) is true, it is still possible that the conclusion (the second sentence) is false. There could, after all, be an unobserved emu out there that can fly.

Deductive arguments, in contrast, are arguments where, if the premises are true, then the conclusions must be true. That is, it is not possible that the premises are true and yet the conclusion is false. Here is an example of a deductive argument:

All humans are mortal. Bob is a human. So, Bob is mortal.

If it is true both that Bob is a human and that all humans are mortal, then it must also be true that Bob is mortal.

Given that it is only in a deductive argument that the truth of one’s premises guarantees the truth of one’s conclusion, it is clearly preferable to infer deductively rather than inductively. The problem, however, is that we have to depend on lots of inductive inferences in order to gain knowledge of the world. At least in the normal course of events, there is no way to infer that all emus are flightless other than to observe lots of emus and then make an inference about what all emus are like on the basis of what one has observed regarding the emus one has seen. An inference of this sort is necessarily inductive.

It is thus crucial that induction is a legitimate way of gaining knowledge, since if it is not, then this would seem to preclude us from gaining lots of the knowledge that we typically take ourselves to have. Our dependence on inductive inferences in this way has been shown to be problematic, however, by a famous argument – due to David Hume (1711–76) – that appears to show that inductive reasoning is unjustified. It is this argument – and some of the key responses to it – that will be the key topic of this unit.

Our guide for this unit will be Chapters 9 and 10 of Pritchard.