A Treatise of Human Nature

Although Hume described mathematics as more reliable than geometry, it is still a “mere probability” (231). Arguments from mathematics still depend on cause and effect and past experience. Even an experienced and knowledgeable person’s arguments might be mistaken or based on wrong information (233-34). Hume acknowledges that this kind of thinking can lead to “total scepticism” (234), where no argument or belief seems certain. Clearly, though, we usually do not think this way. Being skeptical of everything is too hard, so “we retain a degree of belief, which is sufficient for our purpose, either in philosophy or common life” (235-36). For example, asking if our body actually exists is something there is no point in being skeptical about (238). We believe in many basic things simply because we have to.

Even so, Hume asks why we believe in the existence of a physical world, including our own bodies. Specifically, Hume wonders why we have a belief in objects’ continued existence (i.e., the fact they exist even when we are not around to experience them) and their distinct existence (i.e., that they exist at all outside our minds). Hume notes we cannot rely on our senses to answer these questions. After all, our senses only suggest something’s existence when we are around to experience it (239). Nonetheless, each person’s individual sense can only give a part of the picture of an object. Every impression of something is just a perception coming through one or more of our five senses. Still, most people, even children, have no problem believing that there is a world around them. Hume attributes this certainty to the imagination (241). Our imagination absorbs our impressions that appear with constancy (an object appears the same over time) and coherence (an object maintains most or all of its key characteristics despite being changed in some way) (245).

Based on these impressions, the imagination gives us a belief that the outside world exists apart from us and our senses. The imagination does this in two stages. First, we get an impression that an object exists in the same way over time, which Hume describes as the “invariableness and uninterruptedness” (252) of an object. This gives the imagination the idea that an object is an individual thing. Second, the imagination can tell when the impressions of the same object are interrupted, but the impressions are still generally the same. Hume gives the example of looking at the furniture in his room, closing his eyes, and then looking at the exact same furniture again (254).

That said, these are all only impressions in the mind organized through our imagination. According to Hume, we cannot be completely reassured that the world outside our mind truly exists. As a result, Hume believes the only reason we can avoid having too much skepticism about the nature of reality is through “[c]arelessness and in-attention” (268). Both ancient and modern philosophy have failed to address this problem with being certain about the existence of the world around us. Particularly, Hume criticizes the idea from ancient philosophers that everything is made out of either one or several substances. According to these philosophers, differences in the substance, like different colors or shapes, are accidents, meaning these are differences that do not actually change the essence of the substance. Anything that did not fit the idea of substances and accidents were just labeled occult qualities (271-72). Hume dismisses these philosophical ideas as “fictions” (274) invented to explain away reality.

Turning to modern philosophers, Hume is just as critical. For Hume, modern philosophy has been based on the idea that there are primary qualities that exist in the outside world, such as solidness and motion, and secondary qualities that only exist in the mind, like color, taste, heat, and smell (275-76). However, Hume counters that there are no real reasons to distinguish between primary and secondary qualities. There is no explanation for why we should see color as something that is just in the mind, but that our feeling of the solidness of an object represents something real.

Next, Hume tackles another traditional philosophical subject: the divide between the mind and the body, and whether or not there is a soul. In Hume’s mind, the question is pointless because we only have perceptions and no real idea of any substance, much less a soul (282). Then Hume considers the question of whether or not the mind could be conjoined (unified) with the body. He considers this “utterly absurd and contradictory” (283), since it raises too many questions like whether or not a soul would be conjoined to the entire body or just a specific part, and if the soul would stay conjoined if the body is divided. Hume compares the idea of an immaterial soul to the philosophy of the “famous atheist” (289) Spinoza. Spinoza had argued that the universe and everything in it was made of the same simple substance. Just as the idea of an immaterial soul presents the problem of how a soul can be a simple substance capable of complex perceptions, Hume argues Spinoza’s philosophy has a similar problem with complex objects emerging out of a simple substance. In sum, Hume concludes that “the question concerning the substance of the soul is absolutely unintelligible” (298) due to the limitations of our perceptions and on cause and effect.

After dealing with the soul, Hume turns to the self. He sees the self as not “any one impression” (299), but instead as a jumble of ideas and impressions that change over time, basically asserting that there is no such thing as a stable and unified self (300). Also, our understanding of the identity of others, such as plants and animals, is limited. We can rarely know about an object’s identity through uninterrupted observation. Instead, we only know the identity of something through the relations of resemblance and causation.