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The debate between Locke and Leibniz over the sources of knowledge exerted a strong influence on the philosophy of the eighteenth century. The famous Scottish philosopher David Hume came down firmly on the side of Locke. Sense experience, he argued, must be the basis of all knowledge concerning matters of fact or existence. Yet the work of Descartes, as we have seen from the above extract (no. 4), had shown, or seemed to show, that any attempt to base knowledge on sensory experience was wrong-headed, since a series of sceptical arguments could be mounted to call the reliability of the senses into doubt. In our next extract, from Section XII of An Enquiry Concerning Human Understanding (1748), Hume provided a systematic critique of much philosophical scepticism.

First Hume targets the kind of scepticism ‘much inculcated by Descartes’ – the kind of radical doubt entertained as a first step, prior to any science or philosophy. It is impossible, Hume argues, for us consistently to entertain this kind of universal doubt, and even if we could do so, the doubts raised would be entirely incurable. Next Hume targets the kind of scepticism that operates at a later stage, ‘consequent to science and enquiry’ – the kind which uses the supposed results of science and philosophy to cast doubt on our ordinary beliefs about the nature of the world around us. Here Hume draws attention to the power of human nature as against abstract philosophical reasoning: men are carried ‘by a natural instinct to repose faith in their senses’. Hume proceeds to develop this theme, by pointing out that ‘nature is always too strong for principle’. We are bound, when we leave the philosophy class and resume the ‘occupations of common life’, to be entirely immune to the abstract reasonings of the extreme sceptics or ‘Pyrrhonians’:¹ they will ‘vanish like smoke’ when put in opposition to the ‘more powerful principles of our nature’. Finally, Hume goes on to allow the merits of ‘mitigated’ or limited scepticism, which warns us against too readily accepting wild and extravagant doctrines. There is a place for philosophy here, in showing us how to think sensibly, and avoid dogmatism. But we can only do this by ‘limiting our enquiries to such subjects as are best adapted to the narrow capacity of human understanding’; we must realize (as Hume argues elsewhere) that all our beliefs about matters of fact and existence must be ‘founded entirely upon experience’.² Hume here sets up what was to become a highly influential conception of knowledge as something that can operate only within the bounds determined by our nature as human beings.

There is not a greater number of philosophical reasonings, displayed upon any subject, than those which prove the existence of a Deity, and refute the fallacies of atheists; and yet the most religious philosophers still dispute whether any man can be so blinded as to be a speculative atheist. How shall we reconcile these contradictions? The knights-errant, who wandered about to clear the world of dragons and giants, never entertained the least doubt with regard to the existence of these monsters.

The sceptic is another enemy of religion, who naturally provokes the indignation of all divines and graver philosophers; though it is certain that no man ever met with any such absurd creature, or conversed with a man who had no opinion or principle concerning any subject, either of action or speculation. This begets a very natural question: What is meant by a sceptic? And how far it is possible to push these philosophical principles of doubt and uncertainty?

There is a species of scepticism, antecedent to all study and philosophy, which is much inculcated by Descartes and others as a sovereign preservative against error and precipitate judgement. It recommends a universal doubt, not only of all our former opinions and principles, but also of our very faculties; of whose veracity, say they, we must assure ourselves, by a chain of reasoning, deduced from some original principle which cannot possibly be fallacious or deceitful. But neither is there any such original principle which has a prerogative above others, that are self-evident and convincing; nor, if there were, could we advance a step beyond it, but by the use of those very faculties of which we are supposed to be already diffident. The Cartesian doubt, therefore, were it ever possible to be attained by any human creature (as it plainly is not) would be entirely incurable; and no reasoning could ever bring us to a state of assurance and conviction upon any subject.

It must, however, be confessed that this species of scepticism, when more moderate, may be understood in a very reasonable sense, and is a necessary preparative to the study of philosophy, by preserving a proper impartiality in our judgements, and weaning our mind from all those prejudices, which we may have imbibed from education or rash opinion. To begin with clear and self-evident principles, to advance by timorous and sure steps, to review frequently our conclusions, and examine accurately all their consequences (though by these means we shall make both a slow and a short progress in our systems) are the only methods by which we can ever hope to reach truth, and attain a proper stability and certainty in our determinations.

There is another species of scepticism, consequent to science and enquiry, when men are supposed to have discovered either the absolute fallaciousness of their mental faculties, or their unfitness to reach any fixed determination in all those curious subjects of speculation about which they are commonly employed. Even our very senses are brought into dispute by a certain species of philosophers; and the maxims of common life are subjected to the same doubt as the most profound principles or conclusions of metaphysics and theology. As these paradoxical tenets (if they may be called tenets) are to be met with in some philosophers, and the refutation of them in several, they naturally excite our curiosity, and make us enquire into the arguments, on which they may be founded.

I need not insist upon the more trite topics, employed by the sceptics in all ages, against the evidence of sense; such as those which are derived from the imperfection and fallaciousness of our organs, on numberless occasions; the crooked appearance of an oar in water; the various aspects of objects, according to their different distances; the double images which arise from pressing one eye; with many other appearances of a like nature. These sceptical topics, indeed, are only sufficient to prove that the senses alone are not implicitly to be depended on; but that we must correct their evidence by reason, and by considerations, derived from the nature of the medium, the distance of the object, and the disposition of the organ, in order to render them, within their sphere, the proper criteria of truth and falsehood. There are other more profound arguments against the senses, which admit not of so easy a solution.

It seems evident that men are carried, by a natural instinct or prepossession, to repose faith in their senses; and that, without any reasoning, or even almost before the use of reason, we always suppose an external universe, which depends not on our perception, but would exist, though we and every sensible creature were absent or annihilated. Even the animal creation are governed by a like opinion, and preserve this belief of external objects in all their thoughts, designs and actions.

It seems also evident that when men follow this blind and powerful instinct of nature, they always suppose the very images, presented by the senses, to be the external objects, and never entertain any suspicion that the one are nothing but representations of the other. This very table, which we see white, and which we feel hard, is believed to exist, independent of our perception, and to be something external to our mind, which perceives it. Our presence bestows not being on it: our absence does not annihilate it. It preserves its existence uniform and entire, independent of the situation of intelligent beings who perceive or contemplate it.

But this universal and primary opinion of all men is soon destroyed by the slightest philosophy, which teaches us that nothing can ever be present to the mind but an image or perception, and that the senses are only the inlets, through which these images are conveyed, without being able to produce any immediate intercourse between the mind and the object. The table, which we see, seems to diminish, as we remove farther from it; but the real table, which exists independent of us, suffers no alteration: it was, therefore, nothing but its image, which was present to the mind.

These are the obvious dictates of reason; and no man, who reflects, ever doubted that the existences, which we consider, when we say, this house and that tree, are nothing but perceptions in the mind,³ and fleeting copies or representations of other existences, which remain uniform and independent.

So far, then, are we necessitated by reasoning to contradict or depart from the primary instincts of nature, and to embrace a new system with regard to the evidence of our senses. But here philosophy finds herself extremely embarrassed, when she would justify this new system, and obviate the cavils and objections of the sceptics. She can no longer plead the infallible and irresistible instinct of nature: for that led us to a quite different system, which is acknowledged fallible and even erroneous. And to justify this pretended philosophical system, by a chain of clear and convincing argument, or even any appearance of argument, exceeds the power of all human capacity …

The great subverter of Pyrrhonism or the excessive principles of scepticism is action, and employment, and the occupations of common life. These principles may flourish and triumph in the schools, where it is, indeed, difficult, if not impossible, to refute them. But as soon as they leave the shade, and by the presence of the real objects, which actuate our passions and sentiments, are put in opposition to the more powerful principles of our nature, they vanish like smoke, and leave the most determined sceptic in the same condition as other mortals.

The sceptic, therefore, had better keep within his proper sphere, and display those philosophical objections, which arise from more profound researches. Here he seems to have ample matter of triumph; while he justly insists, that all our evidence for any matter of fact, which lies beyond the testimony of sense or memory, is derived entirely from the relation of cause and effect; that we have no other idea of this relation than that of two objects, which have been frequently conjoined together; that we have no argument to convince us, that objects, which have, in our experience, been frequently conjoined, will likewise, in other instances, be conjoined in the same manner; and that nothing leads us to this inference but custom or a certain instinct of our nature;⁴ which it is indeed difficult to resist, but which, like other instincts, may be fallacious and deceitful. While the sceptic insists upon these topics, he shows his force, or rather, indeed, his own and our weakness; and seems, for the time at least, to destroy all assurance and conviction. These arguments might be displayed at greater length, if any durable good or benefit to society could ever be expected to result from them.

For here is the chief and most confounding objection to excessive scepticism, that no durable good can ever result from it, while it remains in its full force and vigour. We need only ask such a sceptic, What his meaning is? And what he proposes by all these curious researches? He is immediately at a loss, and knows not what to answer. A Copernican or Ptolemaic, who supports each his different system of astronomy, may hope to produce a conviction, which will remain constant and durable, with his audience. A Stoic or Epicurean displays principles which may not be durable, but which have an effect on conduct and behaviour. But a Pyrrhonian cannot expect that his philosophy will have any constant influence on the mind, or, if it had, that its influence would be beneficial to society. On the contrary, he must acknowledge, if he will acknowledge anything, that all human life must perish, were his principles universally and steadily to prevail. All discourse, all action would immediately cease; and men remain in a total lethargy, till the necessities of nature, unsatisfied, put an end to their miserable existence. It is true; so fatal an event is very little to be dreaded. Nature is always too strong for principle. And though a Pyrrhonian may throw himself or others into a momentary amazement and confusion by his profound reasonings, the first and most trivial event in life will put to flight all his doubts and scruples, and leave him the same, in every point of action and speculation, with the philosophers of every other sect, or with those who never concerned themselves in any philosophical researches. When he awakes from his dream, he will be the first to join in the laugh against himself, and to confess that all his objections are mere amusement, and can have no other tendency than to show the whimsical condition of mankind, who must act and reason and believe – though they are not able, by their most diligent enquiry, to satisfy themselves concerning the foundation of these operations, or to remove the objections which may be raised against them.

There is indeed a more mitigated scepticism … which may be both durable and useful, and which may in part be the result of this Pyrrhonism, or excessive scepticism, when its undistinguished doubts are in some measure corrected by common sense and reflection. The greater part of mankind are naturally apt to be affirmative and dogmatical in their opinions … But could such dogmatical reasoners become sensible of the strange infirmities of human understanding, even in its more perfect state … such a reflection would naturally inspire them with more modesty and reserve, and diminish their fond opinion of themselves …

Another species of mitigated scepticism which may be of advantage to mankind, and which may be the natural result of Pyrrhonian doubts and scruples, is the limitation of our enquiries to such subjects as best adapted to the narrow capacity of human understanding. The imagination of man is naturally sublime, delighted with whatever is remote and extraordinary, and running without control into the most distant parts of space and time in order to avoid the objects which custom has rendered too familiar to it. A correct judgement observes a contrary method, and avoiding all distant and high enquiries, confines itself to common life, and to such subjects as fall under daily practice and experience, leaving the more sublime topics to the embellishment of poets and orators, or to the arts or priests and politicians. To bring us to so salutary a determination, nothing can be more serviceable than to be once thoroughly convinced of the force of the Pyrrhonian doubt, and of the impossibility that anything but the strong power of natural instinct could free us from it … This narrow limitation, indeed, of our inquiries is in every respect so reasonable that it suffices to make the slightest examination into the natural powers of the human mind, and to compare them with their objects, in order to recommend it to us. We shall then find what are the proper subjects of science and enquiry.