In his Inquiry on Human Understanding, David Hume says that all mental events are either impressions or ideas. Impressions are sensory experiences and emotions. For instance, after you read my paper, you may drink from your blue cup of coffee. While you do, you will see the cup – an impression – taste the coffee’s flavor – an impression – feel its heat – an impression – and upon drinking it, you will immediately feel some visceral satisfaction (before you even say “Mmmm”) with its taste – an impression. Every impression happens instantly and in the moment. You are not reflecting; rather, you are seeing or feeling, independent of thought. So, impressions only happen in the present. They are much more “lively,” as Hume says, than ideas, which are memories or reflections upon these experiences.
All ideas are either simple or complex. Hume’s thoughts about simple ideas constitute the “copy principle.” Simple ideas are not independent thoughts which we create ourselves; rather, they are mere copies of impressions. For instance, you might recall the delicious taste of your coffee – that is a simple idea. You might remember its taste, or its heat – these are also simple ideas. You might also remember the satisfaction you felt when you drank the coffee – this, too, is simple. These are all attempts to copy your past experience, to recreate the impression and enjoy it again. Alas, these memories are nothing like the real thing. They all happen in your mind, not in your eyes or in your mouth or in your heart. Your sensory and emotional faculties are always registering what you feel here and now; they never play re-runs. (If they did, they wouldn’t be doing their jobs!)
Complex ideas, however, are not mere attempts to copy impressions. When you have a complex idea, you synthesize multiple simple ideas into a new and original thought. For example, the thought “That was a good cup of coffee” is a complex idea because you aren’t just recalling the coffee’s taste; you are also assigning the property of goodness to it and stating that it had this property. In fact, any sentence is a complex idea because it is an attempt to synthesize many impressions (in this case, coffee and satisfaction) into a single relationship and value judgment. Just as complex ideas combine simple ideas in order to create sentences, so they can combine simple ideas into imaginary objects like unicorns and golden mountains.
What is most important to remember about Hume’s theory is that the human mind cannot make something from nothing. All complex ideas are dependent on simple ideas, and these are dependent on impressions. Even the person in the Missing Shade of Blue Exception, who produced a simple idea of a new tint of blue, had to be looking at an entire spectrum of other blues to make this observation. All impressions are visceral and independent of thought; once we examine them with our minds, we are creating copies of the moment (simple ideas) and examining them (creating complex ideas). A complex idea which does not have its basis in impressions is “chimerical,” as Leibniz would say.
In the Third Meditation of his Meditations on First Philosophy, Renée Descartes attempts to prove the existence of God. He says that there are three different types of things which exist – “modes” (properties such as color, heat, etc.), physical beings (including humans), and God. Of these three, God is the most perfect (and thus most “real”); physical beings are second, and modes are last. No object can produce ideas which have more reality than they do; otherwise, they would be creating something out of nothing. So a property cannot make us think about a human being or God, and a human being cannot produce in himself or in us the idea of God. Because Descartes is a human being, he is finite and imperfect. He should not be able to conceive of anything which is perfect and infinite, but he reflects on the Lord nonetheless. Furthermore, this idea is not some mere fancy, like a square circle, because whereas a false object like a square circle is impossible to comprehend, Descartes claims to have a better and clearer idea of God than he does of anything else. Therefore God, the only being who is on the same ontological plane as Himself, must have planted this idea inside Descartes. Since this was never a sudden revelation on Descartes’s part, but rather, he had it all along, God must have imprinted this idea in him from the start, so it is an innate idea. Because God has done these things, He must exist.
Would Descartes change his proof if he believed in Hume’s copy principle? At first glance, Hume’s views comport with Descartes’s own. Descartes’s assertion that an object cannot make us think about something which is on a higher ontological status than itself is similar to Hume’s claim that objects with physical existence must cause our simple ideas about these objects. Ultimately, however, the copy principle and Descartes’s proof of God are inconsistent. According to the copy principle, all simple and complex ideas are dependent upon impressions, and all impressions happen to us at a particular point in time. Therefore, we cannot have innate ideas because these sorts of ideas are always inside us and are independent of any impression or sensory experience. Descartes claims, however, that our idea of God is just such an innate idea. Descartes’s conception of God must then be wrong. Either his idea of God has its basis in actual impressions (which he denies), or it is a chimerical supposition which he invented.
Could Descartes amend his proof to agree with the copy principle? Hume says that Descartes’s conception of God is a complex idea, a combination of all good traits (themselves complex ideas) and an extrapolation of them to an infinite degree. Since complex ideas can include false ideas, if Descartes said he had such a complex idea, he would not be proving anything, his claims about the clearness and distinctness of the idea notwithstanding. Furthermore, Descartes rejects this conception of his idea of God; he says that it is impossible for him to separate or remove any of God’s traits, so he must not have put them together in the first place.
Descartes could amend his proof and say that he had had a sudden, beatific vision of God with all his infinite properties, and it had a profound sensory and emotional effect on him. Having this vision would be an impression; remembering it would be a simple idea, and explaining it to others would involve complex ideas. The claim would be unfalsifiable, and it might not even be true, but it would at least agree with the copy principle. To me, it seems possible that one could have such an impression of infinity. I imagine it’s like the feeling one gets when one looks into an abyss.
Descartes could also say that God, because he is all-powerful, can make an exception to the copy principle. If a mysterious shade of blue can thwart Hume, then an all-powerful being must be able to do so, as well. Descartes’s claim is justified if God exists and is all-powerful, but this argument is not very sporting, and it is no longer a proof because the existence of God does not then follow necessarily from our ideas.