Thomas Reid (and David Hume) on Induction, Causality

Introduction

Philosopher Thomas Reid's significance in regard to induction does not derive from his own inductive theory, as in Aristotle's case or Francis Bacon's. In fact, he explicitly states that he has adopted Bacon's method of induction in his Inquiry into the Human Mind, and gives Lord Bacon nothing but the highest praise. What makes Reid so significant is that he understood Hume's criticism of causality (in An Enquiry Concerning Human Understanding), interpreted what it would imply about induction and inductive reasoning, and offered a sort of counterargument to Hume's skeptical doubts.

[I'll abbreviate Hume's An Enquiry Concerning Human Understanding as simply An Enquiry, and Reid's Inquiry into the Human Mind as Inquiry, since I cite those works so much.]

Before we can understand Reid's points about induction, then, we must first understand Hume.

Hume's Inductive Practice: A Brief Aside

David Hume has been seen as the great skeptic of induction, but he didn't think of himself that way. He uses induction on numerous points in his philosophy, including his views on causality, as we will see. He quotes approvingly of some lines of thought in Bacon's method of induction in the section on "miracles" in his Enquiry on the understanding, and utilizes Bacon's notion of a "crucial experiment" (a technical term in Baconian induction) to make a point of his own in section 5 of his An Enquiry into the Sources of Morals.

But Hume certainly was a skeptic of causality; or at the very least, the human capacity to truly discover and understand it. Causality has an intimate relationship to induction, and Bacon, Hume, and Reid all recognize it, each in their own ways. For some reason, Hume thought it necessary to question our knowledge of causality without considering it's impact on the validity of inductive reasoning. And even in this skepticism of causes, he isn't consistent: he names four mechanistic causes as ones that we did in fact discover (elasticity, gravity, cohesive of objects, and moving other objects through mechanical force). In any event, Hume does say things about causality, to which we'll now turn.

Hume on Causality and Human Knowledge

Sensible Qualities, Secret Powers

Let's consider some examples to clarify Hume's positions, examples that Hume and Reid use. A man eats a piece of bread and is thereby nourished; a child touches a flame and experiences pain; a person touches the point of a pin and pricks himself. In cases like these, Hume distinguishes between the sensible qualities of objects that we're aware of in experience, and the secret powers of things that enable things to do or to carry out certain actions: "The bread that I formerly ate nourished me; i.e. a body with such and such sensible qualities did at that time have such and such secret powers." (An Enquiry, Sect 4, Part 2) A flame with such and such sensible qualities (light, warmth) causes a child pain when he tries to handle it, and thus has some kind of secret power.

Hume denies that there's a valid way to sense or reason from the experience of objects' sensible qualities to their secret powers or how those secretive qualities were produced, whether by experience or by rational demonstration:

The qualities of an object that appear to the senses never reveal the causes that produced the object or the effects that it will have; nor can our reason, unaided by experience, ever draw any conclusion about real existence and matters of fact. (An Enquiry, Section 4, Part 1)

Our senses tell us about the colour, weight and consistency [that is, cohesion; not just a pile of grains] of bread; but neither the senses nor reason can ever tell us about the qualities that enable bread to nourish a human body. (An Enquiry, Sect 4, Part 2)

To know about matters of fact, we have to know the relation between cause and effect, and to know about this, we have to consult experience, Hume believes. But all experience seems to show us is (1) the sensible qualities of things and (2) that some secret powers are being made manifest, such as fire burning something else. When asked the question "What are inferences from experience based on?" Hume concludes that reason and the senses must be silent. He goes on to say:

Despite this ignorance of natural powers and forces, however, we always assume that the same sensible qualities will have the same secret powers, and we expect them to have the same effects that we have found them to have in our past experience. (ibid., part 2)

Hume provides the explanation for this soon afterward, in a chain of reasoning central to his criticism of causality and our ability to know it.

The Future Will (Not) Resemble the Past

Hume remarks that all inferences from experience are founded upon the similarities found among things in nature (for instance, several different fires), from which we expect that the effects of these similar things will also be similar. In other words, our inferences from experience really come from this principle: "From causes that appear similar we expect similar effects." (ibid.) After experiencing similar events a number of times, we infer a connection between the sensible qualities and the secret powers, but for Hume this is no different than saying from perceiving what we take to be a cause, we mentally connect it to what we take to be the effect.

Thus Hume discovers the basis for inferences from experience: we assume that the future will resemble the past, and that similar powers will be combined with similar sensible qualities (ibid.). If, however, we presume that the courses of nature may not stay the same, then the past can no longer assist in our knowing about the future, and experience is rendered useless for subsequent inferences. Hume has no problem making this presumption:

However regular the course of things has been, that fact on its own doesn’t prove that the future will also be regular. It’s no use your claiming to have learned the nature of bodies from your past experience. Their secret nature, and consequently all their effects and influence, may change without any change in their sensible qualities. (ibid.)

Loaves of bread, for instance, may have nourished me in the past, but the sensible qualities of tomorrow's loaf won't reveal that it has already been poisoned and thus possesses different, destructive secret powers.

Hume began this inquiry into causality with a question revolving around the nature of knowledge concerning matters of fact; to answer this, he would need to know the foundation of the relation between cause and effect; and to answer that, he would need to know what accounts for our inferences from experience. Throughout the inquiry, Hume has denied that the senses or the faculty of reason can account for matters of fact, real and genuine causes and effects, or the principle that the future will resemble the past. What Hume does think accounts for all of these is "custom," or human habit or instinct.

[Custom] alone is what makes our experience useful to us, and makes us expect future sequences of events to be like ones that have appeared in the past. Without the influence of custom, we would be entirely ignorant of every matter of fact beyond what is immediately present to the memory and senses. (ibid.)

All beliefs about matters of fact or real existence are derived merely from something that is present to the memory or senses, and a customary association of that with some other thing. (ibid.)

So for Hume, our "matters of fact" are really just events, some composed of the sensible qualities we experience, others involving the secret powers latent in objects of nature. When we regularly experience a certain kind of sensible quality (or set of qualities) and observe that it's followed by a certain kind of secret power—when we observe nature's regularity—we come to infer or believe that there's a connection between the two, so that one becomes the cause, the other the effect. This inference or association is something built into the human constitution, a human custom or habit that we can't help engaging in. In this way, we come to believe that the future will resemble the past, and we believe that with the appearance of a cause we should expect an effect.

Reid on Causality

The Inductive Principle

Reid's critical comments on causality and induction appear in his work Inquiry into the Human Mind, in the final section on his chapter about "Seeing." After discussing the forces or powers by which we receive information from others through language, he explains how we gain information about nature, in a set of passages strikingly similar to Hume's.

In human experience, we constantly find things conjoined in the course of nature, and after a number of occasions of similar things, we come to observe the appearance of one as a sign that the other will immediately follow it and we believe that it follows. In other words, the "regularity of nature" or the constancy of nature's laws in the past somehow lead to us to mentally connect what we take to be the cause (or sign) with what we take to be the effect (or thing signified). It is this aspect of our mind's constitution from which we unquestionably assent that the future will resemble the past, or a sort of "...foreknowledge that things you have found conjoined in the past will be conjoined in the future." (Inquiry, Ch. 6 Sec. 24)

This aspect is what Reid calls the "inductive principle": it is the natural force by which we rely on the continuance of nature's laws, without which our "experience" would be useless or even detrimental to our lives. This, Reid observes, is a self-evident truth—an axiom—of our knowledge of nature, including inductive thinking. Like Hume, he initially wonders about how we reached this conviction about nature's regularity.

What makes [the inductive principle] evident to us?[...] True; experience informs us that [natural events] have been conjoined in the past; but no-one has ever had any experience of what is future, and that's our question—How do we come to believe that the future will be like the past? (ibid.)

Reid denies that sense-perception reveals this, as we "...don't perceive in any natural cause any real causality or effectiveness, but only a connection established in the course of nature between [the natural cause] and what is called its 'effect'." (ibid.) Likewise, he denies that reason has any role in making these kind of connections or convictions, and ascribes its origin to instinct:

...[Reason] can't have given rise to this belief [in the first place], for children and idiots [that is, mentally handicapped] have this belief as soon as they know that fire will burn them. So it must be an effect of instinct, not of reason. (ibid.)
[...]
However, I agree with Hume that our belief in the continuance of nature’s law is not derived from reason. It is an instinctive foreknowledge of the operations of nature, very like the foreknowledge of human actions that makes us rely on the testimony of our fellow creatures; and just as we need the latter if we are to be able to receive information from men by language, so we need the former if we are to be able to receive information about nature by means of experience. (ibid.)

The "inductive principle" is something built into the human mind, and according to Reid, it is the basis for all acquired perceptions, for all inductive reasoning, and for all reasoning from analogy. We suddenly have a smell sensation, and later associate it with baked chicken, and due to the inductive principle, we continue to believe that we're perceiving a baked piece of chicken whenever we have a similar smell sensation in the future—this is acquired perception. We play in a body of water and nearly drown, and thanks to the inductive principle, we learn to be more cautious around bodies of water when we encounter them later on; and from this, we come to reason generally that "water can make us drown"--this is inductive reasoning.

Both acquired perception and inductive reasoning rely on the inductive principle, and this instinctual principle (and the surrounding context from which it arises) seems to differ very little from Hume's account of our thinking of cause-and-effect, and its basis in habit or custom. This raises a number of questions:

• Why does Reid think induction is valid, if inductive reasoning isn't even how we reach causal connections?
• Why does he say, "[...]let sound induction alone govern our belief" in chapter 3 of his work, Essays on the Intellectual Powers of Man?
• Why does he accept Bacon's method of induction as a great example of inductive theory, and Newton's work on gravitation and optics as great examples of inductive practice?

What gives?

The Interpretation of Signs

“All our knowledge of nature beyond our original perceptions is acquired by experience, and consists in the interpretation of signs.” (Inquiry, Ch. 6, Sec. 24) Up until now, Reid’s writing on this “interpretation of signs” has focused on an instinctual relating of causes with their effects, differing from Hume’s reasoning concerning our knowledge of causality in only minor details. The key difference lies in how long Reid and Hume think we are governed by this “inductive principle,” this habitual or instinctual reliance on the continued regularity of nature.

Let’s think of Hume and Reid’s “inductive principle” as a kind of mechanical union—just as there's a regularity of nature, of events that we term "causes" and "effects," there is a corresponding regularity in our conception of these things or events, such that the occurrence of one is followed by the belief in the existence of the other. Take a child reciting the number line, for instance: you might ask him to count one to ten, and for him that's what "number line" means, such that if he's prompted to give the number line again, he'll simply count one to ten. He doesn't yet know what those numbers are, but he's associated it with the phrase "number line," as a mechanical union—for him, the existence of one implies the existence of the other.

Hume thinks that this mechanical union is simply how we understand cause-and-effect and matters of fact—there's no escape from it. "These operations of the soul are a kind of natural instinct, which no reasoning or process of the thought and understanding can either produce or prevent." (An Enquiry, Sec. 5, Part 1) But Reid disagrees, and says that, "[this inductive] force, like the force for trust [in people's words], is unlimited in infancy and is gradually restrained and regulated as we grow up." (Inquiry, Ch. 6, Sec. 24) What replaces it, eventually, is reason. Eventually, we learn to stop believing anything that comes out of people's mouths—we learn not to automatically trust them—and we start to reason and examine or analyze what people say. Similarly, we eventually learn not to place our faith in any and all natural events, and gain the ability to reason about the origin and processes of natural events.

Human reason is not infallible: just as we misinterpret what people say sometimes, we can misinterpret what natural events tell us. The comparison between people's words and natural events ends here, however, since Reid believes that nature cannot deceive us, whereas people can lie. Hume believed that reason never touched or governed our beliefs in causes and effects, or generally in matters of fact; Reid implies that it can, if we use our reason to interpret nature.

In effect, this is Reid's counter-argument to Hume: (1) though it is true that we all experience the "inductive principle" or various mechanical unions, it is eventually restrained by our reason, and (2) we can use our reason to learn about true causes and effects, and actual matters of fact.

With that said, how do we learn about these true causes and sound interpretations of nature, in Reid's view?

Induction, and the Language of Nature

The language of nature is what we all study, and the students of it belong to different classes.[...] Philosophers [which includes "scientists" in Reid's time] fill up the top class in this school, and are scholars of the language of nature. (ibid.)

Natural events can be likened to nature's documents, composed of signs, and it is our responsibility as students of nature to learn how to soundly and legitimately interpret them. Animals, children, the mentally challenged, ordinary men, and philosophers/scientists comprise different classes, but experience is our only teacher. Though experience is enlightened by the inductive principle, this alone does not result in sound interpretations of nature: we require tutors. The tutoring and education provided by nature is enough to sustain a savage's manner of life, if he takes the appropriate actions; the tutoring offered by human education, in addition to that of nature, "can make a good citizen, a skillful artisan, or a well bred man." (ibid.) To be able to soundly interpret nature with reasoning, we need not only natural and human education, but the kind of tutoring that could form a "Rousseau, a Bacon, or a Newton," the kind that only reason and reflection can instill. (ibid.)

Reid calls Bacon's work, the Novum Organum (New Instrument), a "grammar of the language of nature," something which displays the rules of inductive reasoning, including the idols or fallacious concepts that cause us to often misinterpret nature. (ibid.) And he considers Newton's Principia Mathemtica (specifically Book 3) and Optics as the best models of inductive reasoning. Bacon formulated the method by which we could properly think inductively, and avoid misinterpreting nature; Newton explained how experimental philosophy could find true causes (and thus make accurate theories), and provided the world with two mechanical theories as evidence, namely his theories on universal gravitation and optics. He says that Newton learned his method of natural philosophy from Bacon's rules of induction, and states something important about induction:

The purpose of all those rules is to teach us to distinguish seeming or apparent connections of things in the course of nature from ones that are real. (ibid.)

For Reid, Bacon and Newton were the valedictorians or magna cum laude of the school of nature, and it is their contributions to human reasoning—what they've said about causality, experimental philosophy, and induction—that is at the heart of his confidence in induction. The accomplishments of these two philosophers are the reasons why he thinks we can truly understand natural facts and causality.