David Hume’s A Treatise of Human Nature (1739) is an extensive investigation of the origin, nature, aims, and limits of human knowledge and understanding. Hume divides the operations of understanding into two kinds: 1) comparisons of ideas, and 2) inferences concerning matters of fact. Hume claims that all knowledge is based upon these two kinds of reasoning and that the most important relation with which understanding is concerned is the relation of cause-and-effect.

Hume argues that all ideas are originally derived from sensory perceptions, and that knowledge is ultimately based on experience rather than on reason. He claims that in order to establish the existence of an object we must either have had a sensory perception of that object or have had a sensory perception of other objects from which the existence of that object may be inferred.

According to Hume, reason alone is not sufficent to establish the existence of an object. Sensory perception of an object, either directly or indirectly, is necessary in order to prove the existence of that object. Furthermore, Hume argues that to have the idea of the existence of an object is the same as having the idea of the object itself.

Hume also contends that reason alone is not sufficient to motivate human action. Reason must excite passion and emotion if it is to have any influence on the motives or actions of the will. Hume claims that reason can prevent the expression of a passion or emotion only by exciting a contrary passion or emotion. However, reason and passion are not necessarily contradictory to each other. Reason may promote the expression of passion rather than prevent or suppress it. Reason may allow passion to provide a motivating force for moral conduct.

According to Hume, passion and reason are not contradictory to each other, unless passion is caused by a false perception of reality or is expressed in a manner that is contradictory to its own aim. Indeed, if reason is to influence moral conduct, reason must either excite a passion for a proper object or must reveal how a passion for a proper object may be expressed.

The Treatise is divided into three volumes or books. Book I is entitled "Of the Understanding," Book II is entitled "Of the Passions," and Book III is entitled "Of Morals." Book I describes the origin of ideas, the distinction between simple and complex ideas, the kinds of relations which may occur between ideas, and the nature and causes of belief. Book II describes the causes of the passions, the objects and aims of the passions, the influence of reason on the passions, and the nature of human liberty and necessity. Book III describes how moral distinctions may arise from differences between the feelings of pleasure and pain, and describes the importance of sympathy as a principle of moral conduct.

Hume argues that perceptions may be of two kinds: 1) impressions, and 2) ideas. Impressions include sensations, passions, and emotions. Sensations are primary or original impressions, while passions and emotions are secondary or reflective impressions. Hume claims that all ideas are originally derived from impressions. Secondary impressions may be derived from primary impressions, and secondary ideas may be derived from primary ideas. Primary ideas are mental images of sensory impressions. The only difference between ideas and impressions is that ideas are 'weaker' or 'less lively' perceptions, while impressions are 'stronger' or 'more lively' perceptions.

Hume explains that perceptions may be simple or complex. Simple perceptions are indivisible, but complex perceptions are divisible. Complex impressions may be derived from simple impressions, and complex ideas may be derived from simple ideas. Simple impressions are the original source of all simple ideas. Every simple idea corresponds to an original simple impression. Primary ideas are images of impressions, and secondary ideas are images of primary ideas.

According to Hume, there are no innate ideas in the human mind, since all ideas are originally derived from impressions. In order for ideas to appear in the mind, there must originally have been some sensory impressions to produce them.

Hume argues that there are three kinds of connections or associations which may occur between ideas: 1) resemblance, 2) contiguity in time or place, and 3) causation. These connections or associations of ideas are mental processes or operations by which simple ideas may be combined into complex ideas such as relations, modes, and substances.

Hume describes seven kinds of relations between objects: 1) resemblance, 2) identity, 3) space and time, 4) quantity or number, 5) quality or degree, 6) contrariety, and 7) cause or effect. Relations, modes, and substances may be combinations of simple ideas which are assembled in variable order and form by the imagination in order to try to explain the true nature of reality.

According to Hume, our ideas of time and space are composed of indivisible parts. If our ideas of time and space were not composed of indivisible parts, then it would be possible for them to be infinitely divisible, and if they were adequate representations of the properties of time and space, then time and space would also be infinitely divisible. If time and space were infinitely divisible, then a moment in time or a position in space could be divided into an infinite number of coexisting moments in time or positions in space, and these coexisting moments or positions would be indistinguishable from each other. Thus, Hume argues that our experience of time consists of a succession of distinct moments and that our experience of space consists of a compound impression of distinct sensations.

Depending on how closely our ideas correspond to sensory or reflective impressions, they may vary as to their degree of clarity or obscurity, distinctness or indistinctness. Hume claims that secondary ideas which are produced by remembering original impressions are 'stronger' and 'livelier' than those which are produced by the imagination. Recent memories are also more likely to be 'strong' and 'lively' than remote memories. As memories become more remote, they lose their 'strength' and 'vivacity,' and are more likely to be influenced by the imagination.

Hume defines a belief as a 'lively' idea which is associated with a present impression.¹ The influence of a belief on other ideas or feelings may be determined by its 'strength' and 'vivacity.' The greater the 'strength' and 'vivacity' of an idea, the greater the influence which it may have on other ideas or feelings. The 'strength' and 'vivacity' of an idea may be determined by how closely it corresponds to the original impression from which it is derived.

The idea of cause and effect as defining the nature of a relation between empirical objects is described by Hume as a fundamental aspect of our reasoning about matters of fact. According to Hume, the idea of cause and effect may be based on ideas about four kinds of relations: 1) contiguity in time or place (i.e. the finding that a cause and an effect are contiguous in time or place), 2) temporal precedence (i.e. the finding that a cause always occurs prior to an effect), 3) constant conjunction (i.e. the finding that a cause and an effect always occur in regular succession), and 4) necessary connection (i.e. the finding that a cause and an effect are necessarily connected with each other).

However, Hume argues that, since every idea of which the mind is capable must originally be derived from a sensory impression, there can be no idea of a necessary connection between a cause and an effect unless there is a corresponding impression from which this idea may be derived. Since we cannot have any sensory impression of a necessary connection between a cause and an effect, the only cause-and-effect relations which we can confirm by experience are those of contiguity in time or place, temporal precedence, and constant conjunction. Thus, the idea of a necessary connection between a cause and an effect may be derived from an impression of reflection, and may only be a representation of an internal impression which does not correspond to any external reality.

According to Hume, we cannot prove that there is a necessary connection between a cause and an effect, or between an impression and a corresponding idea. Thus, we cannot prove that any causal relation between constantly conjoined or regularly successive events is logically necessary. We cannot prove that there is a necessary conection between our perceptions and reality. We cannot prove the existence of an objective world, beyond what we can experience by sensory perception.

Hume’s scepticism applies to his approach to metaphysical and religious questions. For example, he argues that we cannot prove that the universe is necessarily created by God, and that it is only possible for us to confirm by experience that the universe and God are constantly conjoined.

Hume disagrees with the Aristotelian principle that there are four kinds of causes (formal, material, efficient, and final), and says that every cause is efficient, insofar as the idea of efficiency has its source in the impression of a constant conjunction of a cause and an effect. Hume also argues that there is only one kind of necessity, and that there is no difference between moral and physical necessity. He defines necessity as a constant conjunction of objects, this conjunction occurring so regularly and invariably that the idea of one of the objects determines the mind to form the idea of the other(s).²

The impressions which are produced by primary impressions and by primary ideas are classified by Hume as secondary impressions. Secondary impressions are divided by Hume into two kinds: 1) calm (ideas of reflection), and 2) violent (passions and emotions). Passions are also divided by Hume into two kinds: 1) direct, and 2) indirect. Passions arising directly from the sensations of pleasure or pain include: desire, aversion, grief, joy, hope, and despair. Passions arising indirectly from the sensations of pleasure or pain include: pride, humility, ambition, vanity, love, hatred, envy, pity, malice, and generosity.³

According to Hume, our actions are determined by our passions, and not by our powers of reason. Our passions may be excited or quieted by our powers of reason, but reason alone is not sufficent to cause or prevent any form of physical or moral action. The difference between good and evil is not discoverable by reason, but arises from the difference between agreeable and disagreeable impressions or from the difference between pleasure and pain.

In Hume’s theory of the passions, the moral quality of an action is determined by the motives rather than by the consequences of the action. Actions cannot be virtuous unless their motives are virtuous. Virtuous actions cannot be motivated by malevolent or intemperate passions. Every action is motivated by passion, which is the source of its moral quality. Some passions may motivate virtuous actions, while others may motivate non-virtuous actions. The moral quality of an action is determined by whether pleasure or pain is the source of the passion which motivates the action.

Hume argues that there is no sensory impression which can be the source of the idea of a self as a complex system of ideas, and that there is no simple impression from which every idea about the identity of a self may be derived. Hume also argues that the self is nothing but a bundle of perceptions, and that there is no impression which may be attained of the self other than that of a collection of changing perceptions.

Hume emphasizes the importance of sympathy as a principle of human nature, and explains that sympathy may be a source of moral virtues. Sympathy may enable us to express concern for others, and may help us to be more aware of the feelings of others. Sympathy may also be an important factor in the development of moral understanding, and may produce a desire to promote the public good.

¹David Hume, A Treatise of Human Nature, edited by Ernest C. Mossner (London, Penguin Books, 1969). P. 144.
²Ibid., p. 221-2.
³Ibid., p. 328.

Hume, David. A Treatise of Human Nature. Edited by Ernest C. Mossner. London: Penguin Books, 1969.