* Having outlined a theory of mind and causality Hume, always thorough, fleshed it out: underlining the importance of imagination; examining probabilistic thinking; and pointing to the inclination of humans to seek unsupported causes. He capped off the discussion by suggesting that the minds of humans and animals were not fundamentally different.
* With that, Hume ended the outline of his theory of mind, saying that now "it would be very allowable for us to stop our philosophical researches", and that there was no way to "make a single step farther." However, for clarification, he chose to reconsider the same argument from a somewhat different position. To do so, after some labored efforts to define the word "belief" -- which he flatly admitted were futile, mostly because everyone really understood the term -- he again probed into the process of imagination.
Imagination is a critical element of the human mind. The modern American philosopher Daniel Dennett says the core question of intelligence is: "What do I do next?" We have to be able to imagine what we are going to do next to think out the best thing to do. It is a powerful capability:
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Nothing is more free than the imagination of man; and though it cannot exceed that original stock of ideas furnished by the internal and external senses, it has unlimited power of mixing, compounding, separating, and dividing these ideas, in all the varieties of fiction and vision. It can feign a train of events, with all the appearance of reality, ascribe to them a particular time and place, conceive them as existent, and paint them out to itself with every circumstance, that belongs to any historical fact, which it believes with the greatest certainty.
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Most people can see the difference between obvious fantasy and observed reality; we believe the first is false, and the second is true. That distinction of belief is emotional: we feel something is false, we feel something is true. Hume then moved on to the workings of the mind, and its ability to make connections. Hume knew that the brain was the seat of thinking, but he did not realize that it is a massive "neural net" that recognizes patterns, and makes connections. He was, however, able to categorize the connections as recognizing the appearances of objects in the world; the locations and co-locations of those objects; and the causal linkages, if any, of those objects:
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These principles of connexion or association we have reduced to three, namely, Resemblance, Contiguity, and Causation; which are the only bonds that unite our thoughts together, and beget that regular train of reflection or discourse, which, in a greater or less degree, takes place among all mankind.
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If, Hume went on, we see a picture of an absent friend, the resemblance brings up memories of and feelings for that friend. As far as contiguity goes, when we think of our home town, we readily think of everything that we know is in it. Causation is similarly evocative, but only because of the firm connections it establishes in our minds:
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When I throw a piece of dry wood into a fire, my mind is immediately carried to conceive, that it augments, not extinguishes the flame. This transition of thought from the cause to the effect proceeds not from reason. It derives its origin altogether from custom and experience.
... When a sword is levelled at my breast, does not the idea of wound and pain strike me more strongly, than when a glass of wine is presented to me, even though by accident this idea should occur after the appearance of the latter object? But what is there in this whole matter to cause such a strong conception, except only a present object and a customary transition to the idea of another object, which we have been accustomed to conjoin with the former? This is the whole operation of the mind ...
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Once again, there's no real logic involved, it's just that we are accustomed to the fact that things observably work one way and not another. There's nothing else useful that we can do:
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... this operation of the mind, by which we infer like effects from like causes, and vice versa, is so essential to the subsistence of all human creatures, it is not probable, that it could be trusted to the fallacious deductions of our reason, which is slow in its operations; appears not, in any degree, during the first years of infancy; and at best is, in every age and period of human life, extremely liable to error and mistake.
... As nature has taught us the use of our limbs, without giving us the knowledge of the muscles and nerves, by which they are actuated; so has she implanted in us an instinct, which carries forward the thought in a correspondent course to that which she has established among external objects; though we are ignorant of those powers and forces, on which this regular course and succession of objects totally depends.
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BACK_TO_TOP* From there, Hume went on to discuss probabilities in the context of his discussion of cause, beginning with:
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Though there be no such thing as Chance in the world; our ignorance of the real cause of any event has the same influence on the understanding, and begets a like species of belief or opinion.
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There's no such thing as chance because we can, in principle, always trace back a causal chain from any event. Modern science does show that such a causal chain must, sooner or later, come to an end -- but never mind that: we simply don't have enough data, enough observations, to be able to trace back any causal chain indefinitely; or often very far; or sometimes at all.
Yes, as Hume pointed out, we can properly figure out the probabilities of a roll of the dice. Roll one die, then assuming it is fair, we get a value of 1 to 6 on equal probability. However, we haven't logically proven that's true, it's just that experience tells us it is. Given a fair die, it is absurd for us to think that it would do anything else, but it's just like the Sun rising the next morning: we're certain it will, there's no point in assuming it won't, but we still can't prove it will. If there were such a thing as telekinesis, in which people could manipulate a die by simple force of mind, intentionally or not, we couldn't rely on a fair die any longer. Of course, by the same coin, no reliable observations have ever shown there is such a thing as telekinesis.
More significantly, a fair die is an artificial human construct that's built as symmetrically as possible to give an equal probability of falling on any of its six sides. We don't actually find things particularly like dice in nature where we can so neatly figure out probabilities. To be sure, there are many things in our experience, like the rising of the Sun in the morning, where we always expect a certain result:
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There are some causes, which are entirely uniform and constant in producing a particular effect; and no instance has ever yet been found of any failure or irregularity in their operation. Fire has always burned, and water suffocated every human creature: The production of motion by impulse and gravity is an universal law, which has hitherto admitted of no exception.
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However, not all things are entirely uniform and constant. There are many things in our experience in which all we can do is play the odds:
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But there are other causes, which have been found more irregular and uncertain; nor has rhubarb always proved a purge, or opium a soporific to every one, who has taken these medicines.
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We see this often in modern health science, in which the effects of medicines and treatments are assessed in statistical terms, showing merely the likelihood that a medicine will be effective, and the likelihood of it having negative side effects; we can also, hopefully, determine which patients are likely to do well, and which are not. The accuracy of the assessment is proportional to the number of case histories used for data, and it can still be wrong. In most of our activities beyond the routine, particularly those we have little experience in, we end up playing hunches, making bets. We perceive a causal relationship between our actions and what we want to achieve, of course, but we have no guarantee of success.
Unlike dice, where we can calculate the odds of throws, in the real world we generally learn the probability of something happening by tallying up the number of times it has happened in defined circumstances. How do we know the probability of being struck by lightning? By reliably observing how often it happens.
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Though we give the preference to that which has been found most usual, and believe that this effect will exist, we must not overlook the other effects, but must assign to each of them a particular weight and authority, in proportion as we have found it to be more or less frequent.
It is more probable, in almost every country of Europe, that there will be frost sometime in January, than that the weather will continue open throughout that whole month; though this probability varies according to the different climates, and approaches to a certainty in the more northern kingdoms. Here then it seems evident, that, when we transfer the past to the future, in order to determine the effect, which will result from any cause, we transfer all the different events, in the same proportion as they have appeared in the past, and conceive one to have existed a hundred times, for instance, another ten times, and another once.
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In the Northern Hemisphere, experience has shown us that it tends to be cold in January and warm in July; and though we can't say in advance what the weather will be like on any given day, we have a good idea of the range of possibilities.
BACK_TO_TOP* Hume then introduced the perplexing, at least at first sight, notion of "necessary connection":
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When we look about us towards external objects, and consider the operation of causes, we are never able, in a single instance, to discover any power or necessary connexion; any quality, which binds the effect to the cause, and renders the one an infallible consequence of the other. We only find, that the one does actually, in fact, follow the other. The impulse of one billiard-ball is attended with motion in the second. This is the whole that appears to the outward senses. The mind feels no sentiment or inward impression from this succession of objects: Consequently, there is not, in any single, particular instance of cause and effect, any thing which can suggest the idea of power or necessary connexion.
From the first appearance of an object, we never can conjecture what effect will result from it. But were the power or energy of any cause discoverable by the mind, we could foresee the effect, even without experience; and might, at first, pronounce with certainty concerning it, by mere dint of thought and reasoning. In reality, there is no part of matter, that does ever, by its sensible qualities, discover any power or energy, or give us ground to imagine, that it could produce any thing, or be followed by any other object, which we could denominate its effect.
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If all causation is correlation, but not all correlation is causation, how can we tell if correlation is causation or not? Only by observing that the correlation is consistent. It may not be perfectly consistent, simply consistent enough to make it a useful bet. In the absence of any observations that destroy the correlation, we believe there is a causal relationship of some degree of strength. This is, incidentally, the basis for the statistical scheme known as "Bayesian inference" -- in which we know the probabilities of a set of different possible causes for an event from experience, and select from them the cause, or causes, of highest probability on the basis of the evidence. Humans do it instinctively and automatically.
How do we know how billiard balls behave? By observing how they behave. All we know about them is what we observe of them. Having observed them, we create a theory of how they behave, and use that theory to predict how they will work in the future under the same circumstances.
We fully accept that they follow the laws of nature -- but that's a somewhat misleading idea, in that there are no laws of nature, at least in terms of anything like human-made laws. Nothing in our experience denies the reality that things work by regular and perceivable rules, but in the end, all we have is observations. Our theory of the operation of billiard balls is no more valid than the observations on which the theory is based, and the conformance of the predictions of that theory to observations. A theoretical argument, in itself, can prove nothing; its results have to be confirmed or denied by observation.
Anything in the theory that isn't linked to observations is a useful accounting rule at best; irrelevant excess baggage at worst. We could come up with multiple useful theories that are consistent with observations -- though in the end, convenience and economy usually means we focus on one. Any theory can be overthrown by consistent observations that show it isn't correct, forcing us to modify the theory, or come up with a new one.
We know of no necessity that billiard balls must work the way they do. They could work some other way, for all we might know; if they did, we'd just have to change our theories to fit the observations. By the exact same coin, we don't know why they behave the way they do; yes, we can zero in our focus and determine the atomic basis for their behavior, but once we reach the end of observations -- and eventually, we always will -- all we can say is: "That's just the way they work." Again, we don't know why the entire Universe is the way it is, even though we can break its operations down into endless detail.
As Hume pointed out, we don't know exactly how we get our bodies to do things. Our bodies are of central importance in our understanding of causality, since we have to deal with them on an unceasing basis -- getting the body to do something, learning what it does and cannot do, and keeping it in working order. Hume said:
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The motion of our body follows upon the command of our will. Of this we are every moment conscious. But the means, by which this is effected; the energy, by which the will performs so extraordinary an operation; of this we are so far from being immediately conscious, that it must for ever escape our most diligent enquiry.
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We can't tell a toddler how to walk; we just prod and guide her along until she figures it out. If we think to waggle our fingers, we just do it, we don't see a problem with it, and don't worry about how it happens. If we do worry about it -- trying to seek a "necessary connection", "secret connection", the "hidden energy" between thought and action -- we go nowhere:
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... is there any principle in all nature more mysterious than the union of soul with body; by which a supposed spiritual substance acquires such an influence over a material one, that the most refined thought is able to actuate the grossest matter? Were we empowered, by a secret wish, to remove mountains, or control the planets in their orbit; this extensive authority would not be more extraordinary, nor more beyond our comprehension.
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We only know how the body works by our experience with it. Of course some of the functioning of our body, such as heartbeat or digestion, is beyond our direct control. If we suffer an injury that paralyzes, say, an arm, we try to get the arm to work, and nothing happens. The causal relationship between thought and action that we have become accustomed to breaks down:
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We learn the influence of our will from experience alone. And experience only teaches us, how one event constantly follows another; without instructing us in the secret connexion, which binds them together, and renders them inseparable.
... The command of the mind over itself is limited, as well as its command over the body; and these limits are not known by reason, or any acquaintance with the nature of cause and effect, but only by experience and observation, as in all other natural events and in the operation of external objects. Our authority over our sentiments and passions is much weaker than that over our ideas; and even the latter authority is circumscribed within very narrow boundaries.
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There's really no such thing as a necessary connection, there are only observed connections. Hume, incidentally, did himself no favors by speaking of "necessary connections", but not clearly specifying "observed connections" as the alternative, leaving the reader to figure it out -- Hume had his stumbles. In any case, for things that are ordinary to us, we do not concern ourselves with necessary connections. Something happens, and experience tells us what will happen as a consequence:
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The generality of mankind never find any difficulty in accounting for the more common and familiar operations of nature -- such as the descent of heavy bodies, the growth of plants, the generation of animals, or the nourishment of bodies by food: But suppose that, in all these cases, they perceive the very force or energy of the cause, by which it is connected with its effect, and is for ever infallible in its operation. They acquire, by long habit, such a turn of mind, that, upon the appearance of the cause, they immediately expect with assurance its usual attendant, and hardly conceive it possible that any other event could result from it.
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Necessary connections, in contrast, are useless contrivances. When we are confronted with things out of the ordinary, where we do not know the cause, or don't think the cause we see is sufficient to account for the result, then we start dreaming up necessary connections, derived from a leap of false intuition, entirely unsupported by reliable observation:
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It is only on the discovery of extraordinary phaenomena, such as earthquakes, pestilence, and prodigies of any kind, that they find themselves at a loss to assign a proper cause, and to explain the manner in which the effect is produced by it. It is usual for men, in such difficulties, to have recourse to some invisible intelligent principle as the immediate cause of that event which surprises them, and which, they think, cannot be accounted for from the common powers of nature.
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On first reading, Hume's discussion of necessary connection seems like so much annoying hand-waving. Once understood, however, it becomes obvious that people have a tendency to believe in necessary connections -- which aren't actually there. Instead of observing a cause, they make one up and believe it, even though it isn't honestly supported by reliable observation:
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But philosophers, who carry their scrutiny a little farther, immediately perceive that, even in the most familiar events, the energy of the cause is as unintelligible as in the most unusual, and that we only learn by experience the frequent Conjunction of objects, without being ever able to comprehend anything like Connexion between them.
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That's why the idea of necessary connections is hard to explain: they don't really exist. The sciences have occasionally fallen down the rabbit hole of necessary connections -- for example invoking "phlogiston" to account for heat, and an "elan vital (life force)" to account for life. Today, we know that heat is no more or less molecular motion, and life is no more or less biochemistry. The notion of "life force" does persist in modern fantasy fiction and the like; it's part of the human inclination to establish false causal connections.
In the same way, humans retain belief in parapsychology: telepathy, clairvoyance, telekinesis, and other remarkable powers of the mind that cannot be established by any honest investigation. There are also conspiracy tales, in which diverse events are believed to be the work of some massive conspiracy -- but we can't honestly prove it, because the conspiracy murdered all the witnesses. To be sure, all events that occur follow causal paths; but coincidences between them do not establish a "necessary conspiracy" behind them all. If people were so inclined, they could collect sinister things that happened on Christmas and claim they suggested a conspiracy devised by Santa Claus.
Of course, Santa Claus himself is a necessary connection, though only children believe in him. Adults are more ambitious, inclined to believe the all-powerful Deity is the ultimate necessary connection, the "Prime Mover" without which nothing can happen:
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... the true and direct principle of every effect is not any power or force in nature, but a volition of the Supreme Being, who wills that such particular objects should for ever be conjoined with each other. Instead of saying that one billiard-ball moves another by a force which it has derived from the author of nature, it is the Deity himself, they say, who, by a particular volition, moves the second ball, being determined to this operation by the impulse of the first ball, in consequence of those general laws which he has laid down to himself in the government of the universe.
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By this theory everything, absolutely everything is driven by the will of God. It explains everything, while actually explaining nothing; we are no wiser in any specific way whether we believe such a thing or not. Why does the Deity need to be continuously keeping things going? Why can't they just go on their own? The Deity ends up relegated to so much excess baggage:
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... according to these philosophers, every thing is full of God. Not content with the principle, that nothing exists but by his will, that nothing possesses any power but by his concession: They rob nature, and all created beings, of every power, in order to render their dependence on the Deity still more sensible and immediate.
They consider not that, by this theory, they diminish, instead of magnifying, the grandeur of those attributes, which they affect so much to celebrate. It argues surely more power in the Deity to delegate a certain degree of power to inferior creatures than to produce every thing by his own immediate volition. It argues more wisdom to contrive at first the fabric of the world with such perfect foresight that, of itself, and by its proper operation, it may serve all the purposes of providence, than if the great Creator were obliged every moment to adjust its parts, and animate by his breath all the wheels of that stupendous machine.
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Attempting to establish universal power, necessary connection, in the Deity goes nowhere:
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... it seems to me that this theory of the universal energy and operation of the Supreme Being is too bold ever to carry conviction with it to a man, sufficiently apprized of the weakness of human reason, and the narrow limits to which it is confined in all its operations. Though the chain of arguments which conduct to it were ever so logical, there must arise a strong suspicion, if not an absolute assurance, that it has carried us quite beyond the reach of our faculties, when it leads to conclusions so extraordinary, and so remote from common life and experience.
>We are got into fairy land, long ere we have reached the last steps of our theory;< and there we have no reason to trust our common methods of argument, or to think that our usual analogies and probabilities have any authority. Our life is too short to fathom such immense abysses. And however we may flatter ourselves that we are guided, in every step which we take, by a kind of verisimilitude and experience, we may be assured that this fancied experience has no authority when we thus apply it to subjects that lie entirely out of the sphere of experience.
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To exhaust the argument -- which, as Hume admitted, "is already drawn out to too great a length" -- Hume said that we "have sought in vain for an idea of power or necessary connexion ..." There are no necessary connections, just observed connections. We only know how the world works from our observations of it. Again, we cannot see beneath appearances, since appearances are all we have. We can understand in detail how billiard balls work -- but we can't know why they work in one way and not another. We simply observe how the world works, and conform our thinking to reality.
BACK_TO_TOP* Hume finally ended his discussion of cause with yet another recapitulation:
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>All our reasonings concerning matter of fact are founded on a species of Analogy, which leads us to expect from any cause the same events, which we have observed to result from similar causes.< Where the causes are entirely similar, the analogy is perfect, and the inference, drawn from it, is regarded as certain and conclusive: nor does any man ever entertain a doubt, where he sees a piece of iron, that it will have weight and cohesion of parts; as in all other instances, which have ever fallen under his observation.
But where the objects have not so exact a similarity, the analogy is less perfect, and the inference is less conclusive; though still it has some force, in proportion to the degree of similarity and resemblance. The anatomical observations, formed upon one animal, are, by this species of reasoning, extended to all animals; and it is certain, that when the circulation of the blood, for instance, is clearly proved to have place in one creature, as a frog, or fish, it forms a strong presumption, that the same principle has place in all.
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The point of this recapitulation is that the minds of animals unavoidably have similarities to the minds of humans. Animals, like humans, learn causal relationships from experience:
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A horse, that has been accustomed to the field, becomes acquainted with the proper height which he can leap, and will never attempt what exceeds his force and ability. An old greyhound will trust the more fatiguing part of the chace to the younger, and will place himself so as to meet the hare in her doubles; nor are the conjectures, which he forms on this occasion, founded in any thing but his observation and experience.
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Humans exploit the learning capabilities of animals to train them, for example training dogs to recognize their names. There really isn't any logical reasoning involved -- and there isn't any truly logical reasoning involved in humans learning from experience, either, Hume memorably commenting:
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It is impossible, that this inference of the animal can be founded on any process of argument or reasoning, by which he concludes, that like events must follow like objects, and that the course of nature will always be regular in its operations. For if there be in reality any arguments of this nature, they surely lie too abstruse for the observation of such imperfect understandings; since it may well employ the utmost care and attention of a philosophic genius to discover and observe them. Animals, therefore, are not guided in these inferences by reasoning: Neither are children: Neither are the generality of mankind, in their ordinary actions and conclusions: >Neither are philosophers themselves, who, in all the active parts of life, are, in the main, the same with the vulgar, and are governed by the same maxims.<
Nature must have provided some other principle, of more ready, and more general use and application; nor can an operation of such immense consequence in life, as that of inferring effects from causes, be trusted to the uncertain process of reasoning and argumentation. Were this doubtful with regard to men, it seems to admit of no question with regard to the brute creation; and the conclusion being once firmly established in the one, we have a strong presumption, from all the rules of analogy, that it ought to be universally admitted, without any exception or reserve.
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It isn't reasoning; it is, again, "Custom" or "Habit", rooted in instinct, which we share with other animals:
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... the experimental reasoning itself, which we possess in common with beasts, and on which the whole conduct of life depends, is nothing but a species of instinct or mechanical power, that acts in us unknown to ourselves; and in its chief operations, is not directed by any such relations or comparisons of ideas, as are the proper objects of our intellectual faculties. Though the instinct be different, yet still it is an instinct, which teaches a man to avoid the fire; as much as that, which teaches a bird, with such exactness, the art of incubation, and the whole economy and order of its nursery.
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