How does Hume justify belief in an external world?

Thupten asked:

Can you explain how Hume justifies the existence of an external world?

Answer by Geoffrey Klempner

To readers of Hume’s Treatise of Human Nature this might seem like a rather strange question, given that Hume, in his philosophical system of ‘impressions and ideas’, seems to cast the very idea of an external world into doubt. We can’t even say we have the idea of an external world to sceptical about, given that the attempt to define an external object as one which ‘continues’ to exist when not perceived, and exists ‘distinct’ from perception, is flatly contradicted by the only evidence or basis for belief that we have, the nature of our subjective ‘impressions’ (‘On Scepticism With Regard to the Senses’, Book I, part 4, section ii).

Hume’s only solution was to leave such ‘cold, strained speculations’ behind and play a game of backgammon with his friends. At least, so goes the standard account.

But the standard account sits rather badly with Hume’s broad picture of the nature of his investigation. He is putting forward a ‘theory of human nature’, in the spirit of investigators of the time, like Newton with his ‘theory of motion’. Human nature can be studied by the philosopher, in a not dissimilar way to the study of the motions of the planets.

This is the very opposite of a ‘sceptical system of philosophy’. Hume saw his project as a contribution to human knowledge, opening our eyes to the truth about our place in the world, as governed by natural laws, not of motion, but of psychology.

And there is the clue: belief in an external world isn’t capable of rational justification. We just do believe in an external world, and every action we do proves this belief.

However, Hume wasn’t content to leave matters there. The most fascinating part of his philosophy is his ‘theory of fictions’, where he to some extent anticipates Kant in describing the way the human mind ‘constructs’ of external objects by means of the imagination, ideas that cannot be logically justified but which ‘work’ nonetheless. These ‘fictions’, when viewed from the standpoint of reason, have contradictory properties. As ideas, they clearly depend on us, and yet at the very same time they purport to represent ‘objects’ which exist independently.

Hume would say, what this shows is that philosophers have to once and for all learn their place. In place of reason and justification, there is naturalistic explanation. The external world doesn’t need to be ‘justified’.


What is Hegel’s dialectical process composed of?

Ray asked:

What is Hegel’s dialectical process composed of?

Answer by Martin Jenkins

Thought constitutes what is real. What is real is actual and what is actual is real. The real is the living mind of God. Human consciousness is to be unified with the mind of God when it achieves the level of the Absolute Idea. Here, human ‘subjectivity’ is reconciled with ‘Objectivity’ and this goal is undertaken by thought in Dialectic. Hegel’s main works The Phenomenology of Spirit and Encyclopaedia of the Philosophical Sciences outline this movement of collective human consciousness (Geist) towards its reconciliation with the Absolute Idea.

Hegel identified human Understanding as operating within ‘identitarian’ thought: exemplified by the traditional Laws of Logic: Identity [a thing is what it is and not anything else], Non-Contradiction [Cannot simultaneously maintain that it is raining and not raining] and Excluded Middle [either the night is black or it isn’t, there is no in between]. This left Understanding open to limitation, as the phenomena of the world do not limit themselves to such logical criteria. So much so that Understanding in its logical rigidity fell short of fully understanding things.

Thought has thought and thinking as its content. As such, it encounters contradictions [#11 Shorter Logic] in the ‘non-identity’ of its thoughts, in their counterparts. Unlike traditional logic which abhors contradictions, they are welcomed by Hegel. Thought is compelled to overcome such contradictions, to work and find a solution for them. Hence:

‘To see that thought is in its very nature dialectical and that, as Understanding, it must fall into contradiction – the negative of itself – will form one of the main themes of the Logic’. [ibid]

It is speculative Reason that overcomes contradictions by overcoming tensions, contradictions thereby moving thinking onto a cumulatively higher level to the one thus overcome. This is the famous Aufgehoben: speculative reason overcomes the contradiction, superseding both terms and preserving what is positive in both in a higher, progressive synthesis.

In section #79 of the so called Shorter Logic or to give its correct title: The Logic: Part One of the Encyclopedia of the Philosophical Sciences [the other two being Nature and Spirit], Hegel stipulates the three aspects of his Logical Doctrine:

a. The Abstract or that of the Understanding

b. The Dialectical or that of Negative Reason

c. The Speculative or Positive Reason

Abstract Understanding sets out to comprehend subject matter but encounters problems in the guise of contradictions. Reason highlights the limitations of the fixed, either/or thinking of Understanding. This is the Negative moment of the process. The negative moment is simultaneously positive as Speculative Reason provides Positive solutions. The best of the negative and positive moments are combined and preserved in a progressive, synthesis.

I recommend reading the ‘Lord and Bondsman’ in the Phenomenology and ‘Being, Nothing, Becoming’ at the beginning of the Shorter Logic. These exemplify Hegel’s Dialectic.


What does the soul look like?

Blake asked:

What do you believe the soul looks like?

Answer by Geoffrey Klempner

In the early days of film photography, when the workings of a camera and the process of exposure and printing were still a mystery to many people, certain individuals gained notoriety in the Spiritualist world for taking photographs of ‘ectoplasm’.

Funnily enough, the ectoplasm photographed flowing out of the sitter’s head or body looked remarkably like crumpled white bed sheets.

According to the theory, ectoplasm isn’t the soul or spirit itself, but seems to have been conceived as a material which covers or drapes the soul, enabling it to become visible, or semi-visible.

We can smile at this, but I’m not going to venture an opinion on the ectoplasm theory. It might be true. One would need more that a photograph or two to provide convincing evidence. However, the real question is whether the idea of the soul as something you could, in principle, see by some means – a special camera perhaps, or specially designed glasses – makes coherent sense.

Descartes in the Meditations contemptuously dismisses the popular idea of the soul as a, ‘breath of wind, a vapour’, on strict logical grounds. A thinking substance can have none of the properties of extended substance. Extended substance has spatial position. Thinking substance experiences, perceives, thinks, wills etc. Defined as thinking substance, a soul can have neither extension nor spatial position. Nor does the theory of mind-body interaction require this. The locus of interaction (which Descartes hypothesized to be the pineal gland in the brain) is not, literally, the place where the human soul is located. It is merely the place where it ‘acts’, bringing about changes in the physical world, and being affected by physical changes.

Spiritualists don’t have to agree with this, of course. They can insist on the older notion of the soul or spirit as a quasi-physical entity, capable of passing through walls, but also, when the occasion requires it, having the power to tap a table.

Descartes’ response would be the same as his response to materialism as a theory of the soul. I can doubt whether there exists a spatial world without doubting my existence. If I have a soul body or ‘spirit’ located in space, then that would be another kind of ‘extended substance’. In which case, I must have two souls, my Cartesian non-located soul or thinking substance, and my soul body, the substance that gets draped with ectoplasm at seances.

Wittgenstein remarks somewhere that the human face is the best image of the human soul. This is actually in line with popular representations of the soul body or spirit. How would you recognize a loved one’s soul when you saw it unless you could see his/her face? The Cartesian soul, on the other hand, as pure thinking substance cannot have an appearance – or can it? Those who are religious believe that God can see one’s soul. If God can see my soul, surely He knows ‘what it looks like’?

The answer isn’t obscure or contrived: A virtuous soul looks virtuous. An evil soul looks evil. If there exists a God who is able to ‘see’ your soul, then he does so in a similar way to the way you see yourself when you introspect, only without all the concealing layers of self-deception. What does that look like? It’s something everyone does, even if we don’t always see through the lies. There is something it is like to introspect. But it doesn’t look like anything.


Must everything that evolves have intelligence/ consciousness?

Christopher asked:

Doesn’t everything that evolves have to have intelligence/consciousness? In order for an organism to evolve/adapt to its environment so that it ‘knows’ what traits will make it more fit?

I don’t view evolution as some metaphysical entity and I’m trying to avoid any metaphysical explanation like ‘god’. I’m thinking more in terms of Leibniz’s idea of monads existing in all things, because it could be theorized that consciousness exists at an atomic or cellular level, therefore all things containing cells would have consciousness and be capable of evolving. Also, if a cell can grow/change/reproduce on its own, can’t that be considered evolution itself.

Answer by Craig Skinner

There are three different questions here, and I shall deal with each in turn:

1. Doesn’t everything that evolves have to have intelligence/consciousness?

2. Can a cell’s growing be considered evolution?

3. Could all physical entities be conscious?

1. Definitely not. Evolution by natural selection is a mechanical process whereby succeeding generations of an organism fit their environment better. The mechanism is differential reproduction of inherited random variations. Most variants (genetic mutations or recombinations) are neutral, some are detrimental, a few are advantageous. The latter enhance survival and reproduction, thereby spreading in the gene pool which gradually changes (evolves)due to accumulation of beneficial variants. Organisms don’t ‘know’ what traits will make them more fit, all kinds of variants randomly occur, mostly useless or lethal, but those that happen to be advantageous become selected.

The whole process is ‘blind’ and doesn’t need a designer (‘watchmaker’ in Paley’s famous 19th century analogy), hence the title of Dawkins excellent introduction to the topic. Of course the theory itself has matured since Darwin’s day to include molecular details of inheritance, epigenetics, and much else, but I wont go into that.

2. It’s a matter of terminology. Change occurring in all individual members of a species as they age is called ‘development’. Changes occurring in a species over many generations due to accumulation of genetic variants is called ‘evolution’. Thus I developed successively from gamete into embryo, foetus, baby, child, adult, old man. My species and Pan paniscus (chimps) evolved from the same ancestral species.

3. Yes all entities could be conscious. The idea is that consciousness is VERY dim in say electrons or atoms, minimal in plants and most animals, appreciable in mammals, amounting to self-consciousness in humans. As matter gets organized, so its consciousness gets organized too. This ‘panpsychism’ is an alternative to ’emergence’. In the latter view, consciousness is felt to be a ‘higher level’ property emerging when matter is organized in particular complex ways (brains), but is not present in the electrons, atoms, molecules or neurones themselves. Both emergence and panpsychism (favoured by Leibniz as you say, also by Whitehead) are contenders (among others) in attempts to explain consciousness. But whether panpsychism is true or not has no bearing on evolution by natural selection as far as I can see.

There is no need to postulate any ‘striving’ by the elements of the natural world towards increased organization and higher consciousness. I don’t think electrons, for instance, get tired of orbiting in simple atoms for billions of years and strive to be part of more interesting structures such as trees or bees. I know that views about all creation striving to become closer to God, or to reach the omega point, or about the universe becoming self-aware, have been advanced by Whitehead, Teilhard de Chardin, and proponents of the Final Anthropic Principle, but I’m underwhelmed by such views.


Answer by Jürgen Lawrenz

In principle your idea is sound, although you throw steel balls and eggs into one basket as if there was no difference between them.

Cells: Yes. Atoms: No. The first are living entities, the second chemical elements. The latter behave in mechanical, predictable ways; the former have decision-making powers (as primitive as you like, yet they remain utterly disparate from chemical elements).

As far as Leibniz is concerned, you need to be aware of a subtle distinction. His monad (singular) is a merely theoretical entity–a descriptive convenience. After all, he says that all are created at once. What they share is a striving for existence, which ineluctably means collectivisation. They also have the attribute of force common to them, although this force must be understood in four ways to account for the variety of existents: namely active and passive, primitive and derivative. The result of collectivisation depends on the mix of these attributes, and Leibniz constantly stresses that so-called ‘matter’ always has some entelechial force in it – ‘as little as you like’; and this is not alive in any sense. On the other hand all living things contain a quantity (‘as little as you like’) of matter.

But I’ve written a whole book on the subject and don’t feel like compressing what’s important about it into a handful of words that are vulnerable to misunderstanding. If you’re serious about following this up, check out Ch. 4-5 of my book, and especially the diagrammatic representation on p. 120. It’s all there.

Strangely enough, I’ve also written a book on the other aspect of your question, the issue of intelligence in evolution. You might profit from reading (at least) the introduction.

I don’t think there is much literature on this subject, so again: if you’re serious, this is the place to start. Good luck!


Answer by Shaun Williamson

You don’t answer scientific questions by philosophizing or guessing. Leibniz didn’t have the advantage of knowing about Darwin’s theory of Evolution by means of natural selection. You need to study the theory of evolution in detail before you get into the wild theorizing.

Being conscious means having sensory awareness of the world and to have sensory awareness of the world you need sense organs and a nervous system.

Plants e.g. flowers and trees have no sense organs or consciousness but they have still subject to the process of evolution. So the answer to your question is ‘No you don’t need intelligence or consciousness in order to be subject to the laws of evolution’. Things don’t evolve, they are evolved by forces outside their control (well mostly outside their control).

Please study the theory of evolution in detail before you start thinking about it. So far it seems that you simply don’t understand the basic ideas. Things don’t evolve themselves, they are subject to the laws of evolution. thing.


Is Nietzsche an ethical egoist?

Julia asked:

Explain why Nietzsche’s philosophy could be considered a version of ethical egoism, where ethical egoism is the belief that a moral act is one that furthers ones own goals and desires.

Answer by Martin Jenkins

Julia, I suppose Nietzsche’s writings could, to a degree, be described as a variant of ethical egoism. This insofar as he is not writing to address social classes, masses or nations but, individuals. This is reinforced when he disparages collectivism in the guise of ‘the herd’, is dismissive of modern ideas of democracy, equal rights and socialism. Sovereign individuality appears to be what Nietzsche favours when for example, he writes in Thus Spoke Zarathustra that ‘he who does not obey himself, will be commanded. This is the nature of living things’. In other words, know oneself and legislate for oneself accordingly — which is similar to themes in ethical egoism.

However, the second part of the passage where he writes about ‘living things’ is revealing and, I think, moves significantly away from ethical egoism, understood as a liberal, libertarian philosophy operating within the paradigm of human subjectivity, humanism (modernity), to an ontological one. That is, Nietzsche’s philosophy — with its concentration on individuals understood as Ubermensch, Free-Spirits, New Experimenter/philosophers, is not so much concerned with the freedom of the individual but with the nature and fate of human life as bound up with what her termed its ‘higher types’. It moves way from ethical egoism if indeed, it was ever concerned with it.

This is evidenced in #287 of The Will to Power:

‘My philosophy aims at an ordering of rank: not at individualistic morality.’

I consider this text to be a secondary, supporting text. It is not wholeheartedly reliable. Yet what is written in it can sometimes find explicit verification in the published, authorised Nietzsche texts. In Beyond Good & Evil we find Nietzsche’s belief that normal, healthy society is aristocratic. [#259] In Twilight of the Idols, egoism is mentioned and classed as either ascending or descending. With the former, their value is found in that ‘the whole of life advances through them..’ [Expeditions #33] Through them, the human species is enhanced, developed, changed. Precisely how, is not that clear with Nietzsche. What is clear is that he found existing western values to be epiphenomena of a diseased physiology and a corresponding decline of will to power. New attempters/ experimenters/ philosophers would be the source of the revaluation and refutation of such modern values. They would embody strength, health and ordered, comprehensive drives of will to power. Goethe, Napoleon are the examples Nietzsche provides that point towards the type of the New Attempter etc. Beneath the New Experimenters, society would naturally become hierarchical due to the innate difference between people in terms of the degree of will to power that constitutes them.

So Nietzsche’s theory is based on biological grounds in that ultimately, will to power underpins a healthy physiology and this underpins affirmative life valuations. The New attempter-aristocrats are the embodiment of this and they will naturally rule and create. Aristocracy, hierarchy and biologism which are central themes of Nietzsche’s philosophy, are antithetical to ethical egoism as the value of the individual is thereby predetermined by innate predisposition, corresponding social position and ultimately, the caprice of the aristocratic new attempters. Such prescription and restriction is alien to the freedom essential to ethical egoism. So Julia, I don’t think Nietzsche’s philosophy can be considered a variant of ethical egoism.


Can a perfect being create something imperfect?

Christopher asked:

Can a perfect being create something imperfect? Think about it in religious terms. I think that everyone’s conception of god, regardless of specific religion (excluding religions of the ancient world, of course), is in a word that god is perfect, yet everyone views themselves as being imperfect. If god is perfect and created us then shouldn’t we and everything else god created be perfect as well? Maybe the answer just requires a good definition of ‘perfect.’ I’m not sure, but if my proposition is true then the implications of this would require either a change in the concept of god, or of ourselves.

Answer by Jürgen Lawrenz

You fell right into the linguistic trap that lies in waiting for all who think that concept language can solve our philosophical (and other) problems.

Think very carefully about what you understand by a ‘perfect’ being. Keep trying for a while to account for everything that might be embraced by the word ‘perfection’. You might proceed in line with the medieval scholastics, who kept enumerating human qualities and found all of them ‘imperfect’, therefore God would obviously exhibit superior qualities and attributes. But soon you’re going to run out of attributes, there aren’t that many! In addition, you would find that these attributes all have some relation to human attributes. What we don’t know, we can’t talk about: So – what is a ‘perfect’ existent? If such a one existed, we could know nothing about it! It would have a plethora of attributes utterly beyond our puny understanding of perfection.

There is an anthropological explanation for this, if you feel like pursuing it (e.g. Feuerbach, Essence of Christianity). But this does not validate your conclusion. In fact, your conclusion begs all the questions. ‘God’ is a theological (theoretical and metaphysical) conception. Take ‘him’ out of theory and into the world and you no longer have a ‘God’. You end up like Spinoza, who took this line of thinking to its only logical conclusion, that God is the world, and the world is God, and none of us really exist (empirically). Alternatively you have a household god with whom you can hold a conversation once a day, like with a wise old man.

This is not even mentioning that part where you speak of ‘creating’. Pure prejudice. What makes you state this assumption as if it was self-understood? Why should ‘God’ create anything? Isn’t the essence of perfection to be self-sufficient? Erigena taught that God created the world as a material counterpart in order to mirror himself in the myriad of prototypes which he created by actualising himself. But this is already Step 1 towards ‘imperfection’, as you can surely see.

I hope I’ve given you something to think about in earnest. The best thing for you is to stop using the word ‘perfect’ in arguments of this kind, because it is a word without a denotation – except in such limited environments as a ‘perfectly machined ball bearing’ or a ‘perfect (100%) score’ etc. Now you will also see that ‘perfection’ implies totality, which is ipso facto complete, therefore sterile and therefore uncreative.

So if your ‘God’ was truly perfect, ‘he’ would be a self-contradiction. The only creative act possible to ‘him’ would be to make another exactly like himself. You can see that, can’t you?


Answer by Craig Skinner

The concept you speak of is of an absolutely perfect being (omnipotent, omniscient, eternal, infinite, all-loving). But not even such a being can do the logically impossible. And that is why any creation must be imperfect.

The argument is best set out by Leibniz:

It is logically impossible for a perfect being to create something wholly perfect other than itself; for, by the principle of the identity of indiscernibles (if X and Y are exactly identical in all respects they are one and the same thing), a being that was wholly and completely perfect would just be (identical with) god. So, if god and creation are to be genuinely distinct, they must be ‘discernible’ ie creation can’t have all the perfections of god. Elements of creation may be perfect in some respects, but the fact that they can’t be in all respects is already a departure from absolute perfection.

Part of god’s perfection is infinite creativity. A spiritual world of angels, say, is first created. Although eternal and not subject to decay, they are finite beings, and show moral imperfection (witness Satan).

God’s creativity continues. A physical world is created. But this is necessarily subject to entropic decay. And we, being physical, are part of this process; we are mortal, subject to degeneration and all the accidents of an imperfect world.

The only alternative to an imperfect world is for god not to create a world at all.

In short, the presence of evil in the world, far from being an intractable problem for belief in an absolutely perfect god, is entailed by that very belief.

Leibniz’s view as to the metaphysical necessity of evil was famously lampooned by Voltaire in his short novel ‘Candide’ in which the hero witnesses and experiences horrors and suffering but is constantly reassured by the philosopher, Dr Pangloss, that we live in the best of all possible worlds. Dawkins and other militant atheists continue the tradition of blaming God for the evils in the world, but without endorsing as preferable the only alternative, namely no world at all. Dawkins in particular comes across as being angry with god for not existing, wanting him to come out from under that nonexistence cloak and face the music for all the evils of the world for which he is responsible.

So, evil and suffering are inevitable in any world, whether it arises supernaturally or naturally.

Finally, when talking of the religious conception of god, you exclude ‘religions of the ancient world, of course’. How old do they have to be to fall in this category ? Judaism is some 3000 years old, even Christianity is 2000 years old! Mind you, there are no modern worldwide religions. I wonder if there ever will be a new one in this category.


How a good empiricist gets to grips with gravity

Jack asked:

Aristotle, Newton and Einstein proposed radically different explanations of gravity. And yet, according to Hume, they associated their sense experiences together by using the same patterns. According to both Descartes and Hume, they accepted the same fundamental principles of logic. If Hume and Descartes are correct, why did Aristotle, Newton and Einstein disagree about gravity? Is there simply no foundation one can use to determine whether or not Aristotle, Newton or Einstein was correct?

Answer by Craig Skinner

What a very good question, bringing in key issues in both general philosophy and philosophy of science.

The short answer is that the foundation one can use is the scientific method of conjecture and testing. Put simply, we formulate a hypothesis (conjecture) as to how things work, and test it against the world by observation/experiment looking for confirmation (findings support the hypothesis, we can run with it meantime) or refutation (findings rule out the hypothesis, we must amend or replace it).

To deal first with the three hypotheses.


Hypothesis: things move to their natural place.

His conceptual framework was Earth as centre of the world surrounded by moon, sun, planets and stars going round it. He conjectured that things moved to their natural place. A stone, composed of the element earth, fell to Earth; a flame, composed of fire, moved up to the (fiery) heavens. He had no notion of gravity. If we ask why doesn’t the moon fall to Earth, he might say that it isn’t composed of earth (the moon wasn’t known to be a rock then), or that, like the planets/sun/stars, it is constrained in its orbit by a crystalline sphere. Why don’t we see the latter? Ah, it’s invisible. You can already see that the account is made resistant to refutation by what we think of as ad hoc auxiliary hypotheses such as crystalline spheres. And it appeals to purpose (things go to natural places) rather than mechanism. So it’s not a scientific hypothesis.


Hypothesis: things move under the influence of the force of gravity.

His conceptual framework was heliocentric. He saw that the movement of apples, stones, moons, planets, sun, Earth, comets and cannonballs alike could all be explained in the same way, namely as masses moving (as described by his laws of motion) under the influence of a single force (gravity). It was a wonderful synthesis of all motion, on Earth and in the heavens, and its empirical success assured its acceptance. It predicted the exact times of future eclipses and of cometary returns, and in due course Newtonian calculations were good enough to get men to the moon and back. This success rather silenced it’s philosophical critics.

Leibniz complained that, whereas Descartes proposed things stayed in orbit by being caught up in vortices, at least a coherent notion, Newton was as bad as Aristotle, proposing a force mysteriously acting at huge distances. Put simply, how could the moon know that the Earth is there, being 250,000 miles away, and so go round it. If the Earth was snatched away, how could this affect the moon – would it still go round the void where Earth had been? Newton was acutely aware of this “spooky action at a distance” criticism, and made clear he was only describing (modelling we would say these days) reality, and as regards what the force of gravity actually WAS, “framed no hypothesis (“hypothesis non fingo”). Neither did anybody else for 200 years.

So, despite the philosophical conundrum, this account is a good (excellent) scientific hypothesis, and all observations were supportive, save for anomalies in Mercury’s motion, which were just ignored.


Hypothesis: things move freely, acted on by no force, through curved space.

His conceptual framework was the same as Newton’s. In addition, he formulated the Principle of Equivalence: me, standing on Earth subject to downward gravity and feeling upward pressure on my soles is equivalent to me in outer space being accelerated by a force acting up through my soles. Similarly, me falling freely to Earth from a balloon is equivalent to me at rest in outer space – I feel no force. So, in his theory of gravity, a body falling to Earth is subject to no force. It falls freely BUT its trajectory depends on the curvature of space itself, and this depends on mass in that neighbourhood. – mass tells space how to curve, space tells mass how to move, as we can put it.

Einstein’s and Newton’s theories made different predictions as to Mercury’s orbit and as to stellar appearances at the time of a solar eclipse. Observations confirmed Einstein’s view. And no observation contrary to the theory has so far been made. And, a practical note, although Newtonian calculations get us to the moon safely, they are not good enough for accurate satnav location in city streets, the software is Einsteinian.

So, a better theory than Newton’s (which was already an excellent one), and no problem with action-at-a-distance (there is no force so acting), although one worries about whether space curving is philosophically squeaky clean.

But not the last word. Quantum mechanics is equally well established, and the two theories are incompatible at the ultra-micro level. So, much current effort in physics goes into looking for a quantum gravity theory (string theory and loop quantum gravity are contenders).

So, to summarize, the scientific method replaced Aristotle’s view with a better one (Newton’s), the latter has been replaced by an even better one (Einstein’s), and it is likely that a quantum gravity theory will replace that. No theory can ever be PROVEN to be correct (although probably many are).

To deal now with the cognitive part of your question – how come different people, using the same logic (Descartes, Hume), seeing the same things and associating their perceptions using the same patterns (Hume), come up with different explanations of, say, gravity.

Many everyday truths are manifest, such as now it’s autumn, I’m drinking a glass of wine as I type this, etc. But many truths about the world are hidden, not manifest, such as what makes the moon go round the Earth. So we must grope our way towards them by conjecture and testing, and new conjectures can occur by association/ connection of ideas that nobody thought of before. Let’s accept the Humean story that we associate ideas by contiguity, by similarity (resemblance), by cause-and-effect. Newton saw a similarity that nobody had thought of. He saw that the moon and an apple SIMILARLY fall to Earth. In the apple case, the trajectory hits Earth. In the moon case, although forever falling to Earth, this is counterbalanced by its inertial movement tending to carry it off into space, so that it orbits rather than hits the Earth.

Another famous new connection of ideas by similarity was Darwin’s. Everybody knew that plants/animals could be changed by selective breeding (artificial selection), Darwin saw that nature SIMILARLY worked by selective breeding (natural selection). Finally, Einstein’s realization that falling freely to Earth is SIMILAR to being at rest in outer space (Principle of Equivalence) helped him formulate his theory of gravity. In short new links can be made between different conceptual maps, and cognitive neuroscience has gone some way to clarifying the physical (neuronal) basis of this.


Answer by Helier Robinson

Aristotle, Newton, and Einstein were doing philosophy of nature, which later came to be called empirical science. And empirical science is an excellent foundation one can use to determine which was correct. Aristotle was weak on physics (but strong on biology); he thought gravity was simply the desire of things to reach their own place. Newton had the advantage of knowing Galileo’s principle that thins always move in a straight line unless acted upon by an outside force (he made it his first law) and of knowing Kepler’s laws of planetary motion. Also, Galileo had pointed out that the book of nature was written in the language of mathematics; and Newton was one of the greatest mathematicians who ever lived, while Aristotle disliked mathematics.

Notice that Newton did not have an explanation of gravity: he explained motion and forces, velocities and accelerations. His physics is very good for everyday purposes; it fails only under extreme conditions that he could not have known about in his day, such as very high velocity. Einstein improved on Newton, and also explained gravity as being due to the curvature of space-time. The main difference between all three is the amount of known science they had available to them.