In Hume we see the final logical consequences which an empirical theory of knowledge entails. The result is scepticism. We have no certain, self-evident knowledge of anything. Our knowledge is confined to impressions and ideas, and so we are not in a position to assert the existence either of material objects or of spiritual entities. Our notion of causality, that a particular effect is necessarily produced by a particular cause, is the result of our association of ideas, a habitual or customary observation of certain phenomena which appear to have such relations. These apparent relations do not carry with them any necessity or universality. Sensations or impressions are separated from one another and so do not have in them anything universal or necessary. What is open to us is only a probability and no certainty. Particular causes may not produce particular effects. Causality rests on mere instinct or belief. We do not know of any uniformity, regularity or certainty in the working of Nature. Everything becomes a matter of doubt.
We are limited to perceptions and images. When the notion of causality itself is unfounded, how can we be sure that our perceptions are caused by external objects? Though we are accustomed to observe causal relations among our ideas and perceptions, we do not see any ground for supposing this relation between perceptions and objects. What are things when they are divested of the primary and secondary qualities? They are nothing. The only objects known to us are ideas and impressions. We have no right to assume the existence of objects or soul or God from mere ideas or impressions. Where is certainty in causality, which is only a creature of custom or habit? We have to limit ourselves to our world of impressions and ideas and not go beyond this. Even of the true nature of the empirical world, we can say nothing. We know only our ideas which have neither necessity nor universality in them.
For Hume, no metaphysics of reality is possible. He says that we can know nothing of anything real in itself, neither world nor soul nor God. We cannot have therefore a rational cosmology, a rational psychology or a rational theology. We know of no such thing as a world of enduring things or substances. Hume denies the existence of a permanent soul by declaring that we know no soul as an immaterial substance. In fact we know no substances at all, either externally or internally. We know only passing ideas disconnected from one another. When we try to know an immutable soul, what we catch are mere ideas, perceptions, a bundle of thoughts, a mere flux and not anything simple and indivisible. We do not know whether God is, for we have no reason to believe that the universe should have a cause. We cannot infer the existence of God from our minds, for our minds are constantly changing, and so these cannot prove the existence of a God Who is unchanging and eternal.
It will be noticed that though Hume doubts everything and believes that all that we know is of a doubtful nature, he has no doubts regarding the certainty of the truth of his own theories. A consistent sceptic cannot be certain whether what he declares to be the truth has any certainty in it. But it is obvious that a negation of the validity of one's own position would end in an utter confusion of thought. There is no use in saying: 'I doubt the certainty of my views, too'; for here, again, is a certainty that my views may not be certain, or are not certain. So, a sceptic like Hume becomes perforce a dogmatist in regard to his own position. It was the great Descartes who came to the conclusion that the basis of doubt itself cannot be doubted. The doubter cannot doubt that he is or is engaged in a particular mental or physical activity. A self-evident consciousness of an indivisible self is implied in all the enterprises upon which we embark. Through all the arguments of the sceptic there glares the consciousness of self, without which even scepticism cannot be. Who observes the order of sensations, of causal relation,—he is the self. Who associates ideas, who doubts – he is the self. There is an awareness of the observation of the order of sensations, there is awareness of the customary observation of causal relation, there is awareness of doubt, there is awareness of the idea that sensations are discrete in nature – this awareness is the self. Even the fact of a plurality or diversity of sensations cannot be known without a unitary consciousness of self. This truth is too clear and self-evident to need any explanation. The persistent notion of order and regularity, uniformity or unity in Nature, even supposing that this is in mere imagination, is enough implication of the existence of an indivisible self, which has to be identified with God on account of its indivisibility.
Hume says that life would be impossible if we do not believe in causality and regularity or uniformity in Nature. The very notion of the necessity for life and the impossibility of disregarding the uniform laws of life posits as an implication the existence of an immutable consciousness or self. Life has an urge for discovering uniformity; this urge is super-sensuous and demands an acceptance of a uniform and unitary consciousness, in spite of the sceptic's intellectual contention that nothing beyond a plurality of sensations and ideas is known to us. The involuntary urge for recognising system and unity in life and Nature suggests the oneness of existence, which should at once be equated with the oneness of Consciousness.
If, as Hume says, we have not any intuitive notion of a simple indivisible soul, we would not be living beings as we are. But for such a unitary soul we would not feel that we are wholes or integrated personalities. Personality will fall to pieces, every constituent of the personality will drop away in inconceivably minute shreds, but for an indivisible consciousness supporting the personality. There would not be even the disintegrated pieces of personality, in short, nothing but insanity, if an immutable soul were to be consistently and seriously denied. Without a self there would be no consciousness of identity of personality or of a surviving individual. Even the union of ideas in imagination would not be possible without an indivisible consciousness of being. Hume could not speak of even the customary ideas of unity or of relations, but for an indivisible consciousness of self. Without a permanent self, there can be no thoughts, no ideas, no impressions, nothing. But Hume makes the statement that there is belief in the continued existence of objects, a mere belief no doubt, not a certainty. But from where does this belief arise? How is the notion of the continued existence of objects made possible at all? How is even this belief possible? How can there be even an instinct for uniformity and unity? It is not difficult for one to observe that all these notions – those ideas, instincts or beliefs regarding continuity, uniformity and unity – are contained in an indubitable consciousness, which clamours for absolute unity and order everywhere. Does this not suggest that there is an eternal Self which cannot be denied, however much we may try, and which is itself the essence of uniformity and unity? Hume does not seem to have thought over this problem. And how can Hume reconcile his denial of an indivisible self with his theory of the association of ideas in the observation of causal relation? Without some consciousness of unity and organised existence even Hume could not have framed consistent and intelligible ideas in his mind.
The existence of God is not implied merely in our thoughts, for they are changing, and God is accepted to be an unchanging being. True; but God's existence is implied in the implication of the existence of thoughts, implied in our non-mediate awareness of self. In this consciousness of self are comprehended ideas of eternity, infinity and immutability. Further, the notion of God is implied in the notion of the finitude, changefulness and imperfection characteristic of our individualities and of the external visible universe. Hume's contention that our analogy from the finite to the infinite may even warrant the ascription of mortality and physical embodiment to God is totally missing the point in question. Mortality and embodiment are not the essential characteristics of the individuals; their essential nature is consciousness, indivisible and unchangeable, which alone is attributable to the essential nature of God. The self cannot be doubted and so God, too. Hume could not argue or even be without this consciousness which is at once soul and God.
Hume, however, contradicts himself when he believes in the uniformity of Nature as a certainty in calling miracles as violations of the laws of Nature. He thinks that a miracle is incredible, that the interference of Providence in Nature is impossible, for these appear to him to go counter to the established order of the universe. We have, in the Vedanta, the grand truth declared that Nature and God are essentially one and that there is no such thing as a miracle in the sense of an event that contradicts the laws of Nature. We call something a miracle when it transcends the powers of the human faculties of knowledge. Really, there is no such thing as a miracle or a wonder. It is all quite natural to the laws of the universe to operate in that way, though there are many things in Nature which man cannot understand and which Nature sometimes manifests before his eyes. God does not interfere with the way of the world as an external authority, but what we call the work of Providence is really the natural manifestation, in certain particularised ways, necessary for certain particular situations, of the eternal laws of God in Nature, which is His own Body.
Hume's interpretation of the freedom of the will would imply that there is a continuity of self-consciousness, though he denies this in theory. He says that we become responsible for what we do when our actions proceed as effects from our impulses within. But if we are to be sincere followers of his theory, neither free-will nor determinism can have any meaning for us. There cannot be responsibility for action unless there is consciousness of an enduring self, which Hume denies. He says, human volition follows certain psychological laws, but according to his original theory the observed laws are matters of mere custom or association of ideas, and so they cannot be made arguments for attributing responsibility or free-will to man. Further, as Hume himself admits, free-will loses its meaning if we admit that we are perforce made to do an action by our involuntary impulses and emotions or the inward conditions which become responsible for the performance of the action, and which we could not avoid without ourselves becoming different persons. But what endows an action with the characters of the results of a responsible free-will is the consciousness of one's having done it, whether one has actually done it or not.