Hallo, this is Dr. Bin Song in the course of “History of Modern Philosophy” at Washington College.

After reading Rene Descartes’s Meditations, we will learn another very different philosopher, who is representative for the so-called school of British empiricism, in order that you can get a more holistic picture of the development of modern philosophy. This philosopher is David Hume, and the “temperament” of his thought is modest, amicable, and always remains skeptical towards the over-speculation of philosophy in case that the philosophical speculation may fly too far away in one’s solitary thought from the everyday reality and the common sense of human fellows. For students who have learned western Ancient philosophy, you will find the difference between David Hume and Rene Descartes is similar to the one between Aristotle and Plato. Aristotle’s philosophy is quite empirically oriented. He thought human knowledge derives from observation and experience, and to live a good human life, humans need to follow the good customs of society and cultivate good habits so as to become virtuous. All these themes would be repeated by Hume’s empiricist philosophy to a certain extent. Whereas, Plato’s mind was always preoccupied with the world of the so-called “ideas” or “ideals,” just like Rene Descartes, whose Meditations intends to build the entire system of human knowledge from the most basic, allegedly innate ideas within human mind, such as soul, body and God.

In more concrete terms, the difference between Hume and Descartes can be envisioned in the following aspects:

Firstly, rather than treating “metaphysics” as lying in the root of the tree of philosophy and thus ascribing a fundamental role to it, Hume indicates in his writing a great suspicion towards the discipline of “metaphysics” in general. He thinks scholars’ discussions on metaphysical issues (such as the existence of God, the ultimate power and secret of nature, the essential and immortal nature of soul, the interaction between a thinking mind and an extended body, and other extravagantly abstract topics prevalent in modern philosophy) barely produced any certain result, and the debate on these issues seemed to be endless, if not desperately fruitless. In comparison with the very solid development of natural science in the era of Isaac Newton, who was both a compatriot and a slightly elder contemporary of Hume, Hume thinks that the discipline of metaphysics, as indicated by its poor performance in the early modern era, is deeply flawed. In order to rectify these flaws, Hume thinks that the primary task for philosophers is to imitate what Newton has achieved in natural science, and hence, to investigate the more fundamental topic of “human nature,” particularly the operations of human mind, before diverting our energy into those endless metaphysical debates. Hume’s suggestion is that through investigating the operations of human mind, we will get to know how human ideas are formed from their empirical origins, and how human mind associates these ideas so as to produce knowledge on different subjects. If we can pin down the origins of all the extravagantly abstract ideas in metaphysics, then, the language we use to discuss metaphysics will be more clear and we will therefore be able to grasp the limit of human knowledge lest our metaphysical thinking would have become a sheer speculation doomed into on-going, yet fruitless controversies and debates. (About Hume’s view on metaphysics, please refer to the assigned reading “Inquiry Concerning Human Understanding,” Section 1).

Secondly, a pattern of western philosophy is that idealistic thinkers normally quite cherish the distinctive value of mathematics. For them, such as Plato and Descartes, the empirical world cannot be where mathematical ideas derive, since all empirical knowledge lacks the universality and certainty of what mathematical reasoning can live up to. However, humans cannot arbitrarily manipulate mathematical ideas either, since mathematical objects have their stable traits and inviolate laws which human mind must obey. For Descartes, the two aforementioned reasons, viz., the non-empirical and the objective natures of mathematical knowledge, lead to his conclusion that fundamental mathematical ideas, such as size, magnitude, number, and in one word, “extension” of body, are innate. They are the blueprint of divine creation, which God imprints into human mind so as to inform humans of the essence of bodies which are also created by the same almighty God.

However, for Hume, just like for Aristotle, mathematical ideas do not occupy such a distinctive position in the world of ideas of human mind. Hume distinguishes all human perceptions into two groups according to their degrees of vivacity and strength: impressions and ideas. “Impressions” are those raw, vivid perceptions of the world which humans acquire from outer sense (such as vision, hearing, smell, etc.) and inner sense (such as our feeling of hunger, thirsty, pleasure, pain, etc.). However, when these raw impressions are stored into memory, revived in imagination or abstracted in intelligence, they will lose certain degrees of vivacity and turn into “ideas.” For Hume, all ideas, including mathematical ideas, derive from impressions, and thus, have an empirical origin. It seems that human mind can work on mathematical ideas alone in separation from the empirical world; however, according to Hume, this is because once abstracted from their empirical origins, the connection of mathematical ideas can be investigated according to the logical law of non-contradiction. Even if it seems we can acquire much knowledge of mathematical objects in reliance upon the work of human mind alone, the knowledge is just about the relation of “ideas,” and whether the knowledge can be applied to the everyday empirical world would still depend upon experience and observations. In other words, for Hume, the ideas of math derive from empirically given impressions, their relationship can be investigated by human mind alone, but whether mathematical ideas can be applied in the empirical world would still depend empirically. In a nutshell, there is no innate idea in the Cartesian sense, and all human knowledge derive ultimately from observation and experience, which is definitely a very different stance from Rene Descartes.

Thirdly, and most importantly, since experience and observation, rather than reason, plays the ultimately prominent role in the development of human knowledge, compared with Descartes, Hume also furnishes a much higher evaluation of the role of “customs and habits” in epistemology. As indicated by last unit’s reading, the hyperbolic method of doubt which Descartes uses in his Meditation I targeted customs and habits of human cognition that he inherited from his medieval scholastic background. For Descartes, nothing is more urgent to overthrow all those old ideas and beliefs in order to obtain an entirely new, and absolutely secured foundation of human knowledge. However, with a purpose of highlighting the irreplaceable roles of custom and habit in human knowledge, Hume asked an extremely consequential question for the further development of modern philosophy and for our general understanding of scientific knowledge. And the question is: on what basis shall we infer that an effect will be produced from a cause given our past observation of the constant conjunction of the two events identified respectively as a cause and an effect? For instance, we observed constantly in the past the rising of the sun can lead to the rising temperature of a stone; however, if we conclude “the rising of the sun causes the rising temperature of a stone” as a piece of knowledge, it will imply that in the future, the rising of the sun would always cause the rising temperature of the stone. But Hume’s question is that: how can we infer a future state of the worldly phenomenon based upon our past perception?

Hume does not think “reason,” in the form of demonstration which deduces one consequence from its premise according to the logical law of non-contradiction, can play any role in the aforementioned inference. This is because if the rising sun does not cause the rising temperature of the same stone in any future event, this does not bring any self-contradiction to our ideas. In other words, a different effect from a given cause is entirely possible, and thus, it is not self-contradictory to connect “a rising sun” to “the non-rising temperature of the stone.” In other words, the so-called causal reasoning is actually about matters of fact, which is different from the one about the relationship of ideas, and hence, it must be operated upon a completely different mechanism from the latter.

After surveying all possible answers to the above question, Hume concludes that it is nothing other than “custom and habit” that incline us to infer a stable repetition of future events from our past observation of the same events so as to develop our knowledge of causality and matters of fact. In Hume’s words, “After the constant conjunction of two objects – heat and flame, for instance, or weight and solidity – sheer habit makes us expect the one when we experience the other.” (pp.20, Inquiry Concerning Human Understanding.)

So, according to Hume, there is no secrete power or ultimate cause of nature which guarantees that nature will always proceed according to a fixed, unchangeable set of laws. Instead, Human knowledge of nature, viz., matters of fact in Hume’s words, solely and entirely derives from our observation and experience of the natural world, and thus, it is customs and habits, rather than any pre-established metaphysical reasoning, that help humans to know and adapt themselves to the constantly changing world with its discoverable, falsifiable and perfectible laws of causality. In other words, Hume’s “skepticism” employs customs and habits to remain suspicious towards metaphysical reasoning, while Descartes employs metaphysical reasoning to remain suspicious towards customs and habits. Shall we find a difference between Hume and Descartes more striking than this? I bet it would be very hard.

Nevertheless, despite that Hume’s philosophy can be read in a contrasting manner from Descartes in the above multiple aspects, Hume is surely still a “modern” thinker. His admiration towards Isaac Newton speaks to a common commitment among modern philosophers to the utilization of scientific method, the one of reducing complex issues into simple ones similar to what Descartes has articulated in the Discourse of Method, in the investigation of human mind. Also, Hume’s extraordinary work upon the study of the operations of mind represents another distinctive trait of modern philosophy which we analyzed before: its unusually intensive focus upon the subjective world of human mind. In fact, Hume is thought of as a pioneer of the modern discipline of psychology, and his study on the law of the association of ideas and the related moral philosophy which focuses upon sympathy continually generates great impact upon the study of psychology and the practice of psychotherapy even today.

Based upon the illustrated differences and similarity between Hume and Descartes, I would like to characterize Hume as a rebellious sibling of Descartes in the same family of modern thinkers.