01 September 2014

Essences, natures, and identies, oh my!

Ed Feser, uber-Catholic conservative philosopher extraordinaire, recommends a book by a chap named David Oderberg called Real Essentialism, in which the author argues that 'essences', 'natures', and 'identities' are literally real properties of things. The book's been recommended to me in several of my discussions with Christians who view themselves as 'Thomists', though I'm confident that given the $118 price tag, most if not all of those people are just recommending it because Ed Feser recommends it and not because they've actually been bothered to read it. But while I maintain a strict policy of "I'll read the book you're recommending if you buy it for me or loan it me", its central thesis — that those supposedly 'metaphysical' properties are literally real, existing independently of the human mind — is a central point of contention in my many conversations with Thomists.

It can be difficult to debate Thomists, precisely because they adopt a highly esoteric lexicon and apparently idiosyncratic definitions of their key terms that are not always obvious. For example, a Thomist might tell you that you cannot so much as think about a thing without thinking about its identity — the very thing which allows us to distinguish one thing from another. And surely, we all agree that our brains can and do require us to categorize things and make distinctions between them in order to coherently think about them. But what the interlocutor may miss is that the Thomist is saying more than that — the Thomist is claiming not just that our brains organize patterns into discrete categories and objects, but that objects have, independently of the brain, the property of identity.

This is a subtle but critical distinction. Because surely, again, we all agree that if tomorrow all humans were wiped off the face of the Earth, cats (for example) would still be cats — that is, they would retain the amalgam of physical properties that our brains categorize as simply "cats", even there wouldn't be anyone around to say, "Hey, that's a cat!" But does it follow, then, that cats have a property of identity that makes them cats?

The silliness of such a proposition can possibly be illuminated by looking at things in evolutionary terms. Let's take a mountain. Mountains are formed, if I remember grade school geology correctly, when massive tectonic plates press against each other, forcing the earth to slowly rise over eons. I wonder how the Thomist might think about this, then — at what point does the earth have the property of identity of a mountain, versus just being a really big hill or a giant pile of rocks? If we were to watch a computer simulation of mountain formation, sped up so something like Everest formed in a few minutes, would the Thomist be able to pause the video at the exact right moment and say "There! Now it has the property of identity of a mountain!" Where might this property of identity have come from? Did it evolve from some lower-order property, or did God just put it there?

This line of thinking extends to other terms integral to the Thomistic view, like essence and nature. From my reading, the distinction between identity and essence seems a bit fuzzy, but to use the example of the cat again, we can say that the 'essence of cat' is whatever it is that makes it a cat. A cat has some sort of 'catness' — its material form and the properties that arise from that material form. Try to wrap your head around this convoluted explanation:
The essence of material substances is composed of substantial form and prime matter. Substantial form is the source of the specific identity or identity as a species, as a human being, as a dog, etc. Prime matter is pure potentiality to be specified, determined, activated by the form. It is the principle of individuation: it multiplies the form and accounts for diversity within the unity of the form or species by receiving and restricting the form to “this” material subject–the possibilities of the species are not exhausted by an individual. For example, “humanity”is  multiplied into “many human individuals” by matter.
This illustrates why conversing with Thomists is often confusing; the term 'prime matter' here is not being used to refer to 'matter' in the physical sense (that's the 'substantial form', or the 'form' — a cat, in this instance — that the substance takes). The IEP clarifies it a bit:
If we think of matter as without any form, we come to the notion of prime matter, and this is a type of matter that is totally unformed, pure materiality itself
This is the point when any rational person should want to say Whoa, hold up. "Pure materiality"? "Matter that is totally unformed"? What does that even mean? Suffice to say that no person has ever observed unformed matter. The IEP goes on to explain... sort of:
prime matter, as pure potency, cannot in fact express any concrete mode of being, since as pure potency is does not exist except as potency. Thus, prime matter is not a thing actually existing, since it has no principle of act rendering it actually existing.
Here we get to the central confusion that Thomists trip over — they inadvertently blur the distinction between conceptual abstractions and literally real objects. By the description here in IEP, 'prime matter' by definition doesn't exist; it's just an abstraction of the concept of matter. And yet somehow, on the Thomistic view, this 'pure potentiality', which doesn't actually exist, is somehow able to be acted upon — specified, determined, and activated by the 'form' of things that do exist. To which I have only one reaction:

How does something non-existent interact with existent things, or vice versa? Well, that's a whole other pile of confusion because, according to the Thomist, existence is just a property that something does or doesn't have. The SEP clarifies:
While [material substances] exist, their existing is not what they are. Thomas accepts from Boethius that it is self-evident that what a thing is and its existing differ (diversum est esse et id quod est).
Yes, they differ, in our minds. In reality, things cannot have properties if they do not exist, and things especially cannot interact with other things or be acted upon if they do not exist. Cognitive abstractions do not have properties, but abstractions of properties. This is how we know, for example, that unicorns are imaginary and not real. Unicorns don't have the property of being equines, of having horns, of being able to traverse rainbows, etc., because unicorns are fucking imaginary. Rather, unicorns have the conceptual properties of being equines, having horns, etc.

And that's the rub with Thomism. Thomists take things like identity, essence, nature, 'prime matter' and potentiality to be literally real properties of the external world, independently of human minds. But at every turn, we can see that we have no reason whatsoever to think that any of these 'metaphysical' properties are anything more than conceptual constructs. There's no reason for us to think that the concept of "cat" is anything more than a useful categorization of our brains for a particular arrangement of matter; we have no reason to think that there exists any such a thing as the identity, essence, or nature of a cat independently of our minds. 

To reiterate, this is of course not to say that cats don't have distinct properties, or that cats would cease to be cats if humans suddenly disappeared. But we can reject the Thomistic metaphysical gobbledygook on the principle of parsimony — the notion that cats have a distinct, non-physical property of 'catness' (their 'essence'), for example, is completely superfluous to our understanding and description of what a cat is. We can have a fully accurate, useful description of the animal simply by recognizing it as an amalgam of physical properties which our brains categorize in a particular way, and nothing more. There is no need to postulate any extra non-physical or 'metaphysical' properties to understand what a cat is, why it behaves as it does, or what it evolved from. Since the assumption of the existence of such things is not essential to our description or understanding of cats, we can discard it. We don't even have to demonstrate its falsity — i.e., somehow 'disprove' the existence of those metaphysical properties — we can simply discard them as superfluous and thus meaningless.

In fact, the whole proposition of metaphysical properties just complicates our understanding of the physical world. We know, for example, that the domestic dog evolved from wild wolves. Yes, even Binky the Pekingese is an evolutionary descendent of the mighty wolf, and even science-denying IDers and creationists will not dispute this fact. So at what point does the 'essence of wolf' become 'essence of domestic dog'? This was a gradual process, taking tens of thousands of years as wolves lived on the edges of human settlements; more docile wolves were artificially selected by humans and, over the millennia, became the companion dogs we know and love today. Where does the essence of one species end, and the other begin? How do these essences and substances interact with one another, particularly if they are (in the case of prime matter) non-existent? At what point does a population of wolves take on the metaphysical property of 'dogness'? None of this terminology illuminates anything at all about what these creatures are or the process of evolution itself; it unnecessarily complicates the issue by layering on superfluous and unjustified assumptions.

Aquinas, and his forebearer Aristotle, did not know about the evolution of species nor about the eons over which the earth slowly changed. They had no concept of quantum superposition or indeterminacy. The idea of emergent properties was foreign to them. They seemed to think that something was either this or that, clearly and completely distinct and precisely the way God made it to be. Similarly, they did not understand the mind like we do today; they did not understand that concepts are neural structures in the brain, and not necessarily representative of actual things. And that is why their metaphysical musings are irrelevant; they were a failed science, an ambitious but hopelessly hamstrung attempt to make sense of the world around them and, more importantly, backwards-rationalize the existence of the God they assumed exists. But we no longer have any reason to take their claims about reality seriously, much less take seriously any conclusions about the divine that they hoped to gleam from their metaphysics. Thomism can go to the dustbin, with the rest of metaphysics.

21 August 2014

On the meaning of life

Steven Jake had a post over at his blog The Christian Agnostic in which he quotes Thom Stark in an exchange with John Loftus on the nature of faith and the meaning of life. You can read the full post at Steven's blog, but my comment ended up being long enough that I thought I'd just repost it here.

And, just to shamelessly plug myself a bit, I discussed these issues not long ago in a post On death and dying.


Why is the alleviation of human suffering the right thing to do? If the universe is a cosmic accident (and it may very well be just that), I can’t figure out why human beings should have impetus to behave morally, other than when it helps us to preserve ourselves or our species or to make us happier in some way. When morality conflicts with self-preservation or self-gratification, I just don’t know why morality should win out. [Stark]

I confess that even in my days as a Christian, this type of view (which is pretty standard boilerplate for religion, in my estimation) didn't make much sense to me.What difference does it make whether the universe has some grand cosmic purpose? How does that change what I find meaningful in my life or what my moral imperatives are?

My life is meaningful to me in large part because it is so rare and special, an infinitesimal spec of space and time in an incomprehensibly vast universe that is almost entirely a lifeless void, one that contains more black holes than it does humans that have ever existed. Whatever this life of mine is, one thing's for sure – it is mine to live. Regardless of how I live my life, I'll almost certainly be soon forgotten, and the universe will continue on without me, just as it got along fine without me for billions of years prior to my brief existence. If there is some grand cosmic meaning, it's certainly well-hidden beneath our utter cosmic insignificance.

So, I have this time to make of my life what I can. I can try to live a good life, to enjoy life's pleasures great and small, to learn and grow, to pick myself up when I stumble. I can enjoy a twilight walk with my fiance, or wrestle with our new puppy, lose myself in guitar for hours, or expand my mind with literature or science. The world is at my fingertips waiting to be experienced and discovered, and that is enough for me. I don't need to be told I'm a special snowflake, that all this was put here with me in mind. I make my life's meaning for myself.

Why be good, then? Because I do not exist in a vacuum. I am an interdependent conscious creature living in a cooperative social hierarchy. I do not have the luxury of moral autonomy, because my actions impact the well-being of others and because, as we all are, I'm dependent on the cooperation of others for my own needs. I'll never meet the farmer whose crops have fed me or the people who logged the wood that built my house, but I can recognize that if I don't wish to respect others' needs and interests, others have no reason to respect my own. And again, that is enough for me. I do not need to be told there are eternal consequences for my actions, because I can see the consequences of them in the here and now. How many cheating spouses never felt the joy of truly loving and trusting their partner? How many oppressed and marginalized people could have been great leaders, doctors, teachers, or innovators?

To me, the finality of death or the relative brevity of my existence is not depressing; what's depressing is the thought that this life is not good enough. There can't just be the wondrous universe — there must be gods and angels and magic, too. There can't just be this life — there must be something more, something that never ends, in which I'm free from failure, from the pain and losses that have allowed me to grow and appreciate the time I have, from new beginnings and from bittersweet endings. It's like a book that never ends, or the perfect crescendo of a piece of music that goes on forever. Those things are meaningful to us precisely because they are so fleeting and precious. I don't need or want eternal life. I don't need or want the promise of eternal reward or the pithy threat of eternal torment. My life is meaningful to me because it's all I have.

The irony is that Thom seems to recognize this in his last paragraph. If there is some grand cosmic purpose to our existence, no one seems to have it figured out. Sure, some people claim they have, but really all they've done is declare meaning for themselves. So whether you believe, as I do, that death is the end and life is what we make of it, or that we are reaching for some elusive transcendent purpose that will outlive our Earthly bodies and indeed the universe itself, we're all ultimately forced to do the same thing — find meaning for ourselves. I suppose where I part ways with Thom is his phrase "and hope we get it right". I don't believe there is a "right way" for us to exist. There are better ways and worse ways, but only we can decide what is right for us both individually and collectively. I neither need nor care whether I "get it right", beyond the simplest interpretation of the phrase — that I've spent my life sharing in happiness with others, that I've appreciated the beauty and wonder of the universe, that I've done my small part to make the world a better place for the many others who are like me in our shared humanity, and that when I go, I go smiling back on a life well-lived.

You know that pain and guilt can't be taken away with a wave of a magic wand. They're the things we carry with us, the things that make us who we are. If we lose them, we lose ourselves. I don't want my pain taken away! I need my pain! - Captain Kirk

There's a movie coming out about Stephen Hawking, and it looks fabulous

I enjoyed 2009's Creation, about Charles Darwin, quite a bit. And now another renown and influential scientist is being graced with a biopic that, by the looks of it, could end up being pretty damn good.

18 August 2014

Thoughts on the Ice Bucket Challenge

ALS is a nasty disease. Two people I greatly admire — Stephen Hawking and Jason Becker — have lived with the disease for many years. To most, it's not so kind. Years ago I trained a group of young people whose job it was to care for a wealthy man in his 50s who had ALS. I once asked one of the nurses if she thought she could live with the disease, and she bluntly said, "I would rather die".

If you're not familiar with the heavily viral "Ice Bucket Challenge", the gist is that someone gets a bucket of ice water dumped over them, pledges to donate to help ALS research, and then 'tags' several friends to repeat the challenge. Given its focus on social networking and its novelty, it's been hugely successful in raising money for ALS research. That's a good thing, right?

Yes and no. Raising money to fund treatment for a nasty disease is certainly a good thing. But as William MacAskill – a researcher in moral philosophy at Cambridge – pointed out, there's a problem of 'funding cannibalism'. He notes,
Because people on average are limited in how much they’re willing to donate to good causes, if someone donates $100 to the ALS Association, he or she will likely donate less to other charities.
It's also worth noting that in most of the videos I've seen, no reference is made to whom a charitable donation is supposed to be given, and I'd be willing to be that plenty of people participated without making any donations just for the nebulous effect of "raising awareness". Raising awareness about ALS accomplishes little without action and, more importantly, long-term commitment.

I'm not in with the cynical crowd who asks, "What does dumping a bucket of ice water over your head have to do with ALS?", because the answer is, "About as much as running a 5k has to do with breast cancer." Lots of charities create novelty events to raise money, but as MacAskill argues, this isn't a good long-term solution:
[...] competitive fundraising ultimately destroys value for the social sector as a whole. We should not reward people for minor acts of altruism, when they could have done so much more, because doing so creates a culture where the correct response to the existence of preventable death and suffering is to give some pocket change.
[...] Rather than making a small donation to a charity you’ve barely heard of, you could make a commitment to find out which charities are most cost-effective, and to set up an ongoing commitment to those charities that you conclude do the most good with your donations. Or you could publicly pledge to give a proportion of your income.
These would be meaningful behavior changes: they would be structural changes to how you live your life; and you could express them as the first step towards making altruism part of your identity. No doubt that, if we ran such campaigns, the number of people who would do these actions would be smaller, but in the long term the total impact would be far larger.

For my part, I generally decline solicitations to give to charity as I already sponsor a charity I think is important, and my donations are budgeted out of my regular income. That's not to say I can't forgo a dinner out for a one-off donation to a good cause, but I generally dislike doing that for the same reason – my charitable donations are budgeted, so if I gave every time I was solicited it'd cut into my regular charity budget.

The Ice Bucket Challenge has raised over $10 million for ALS research, and that's a good thing, but we should all take a moment to consider longer-term commitments to causes we find meaningful.

13 August 2014

The militarization of police is one of the most important civil rights issues America has ever faced

There's an old saying that when you're a hammer, everything looks like a nail. So what happens when, despite violent crime being at a 44-year low, the Department of Defense is allowing local law enforcement agencies to acquire its surplus tactical armament? The answer is precisely what happened in Ferguson, MO — police confronted unarmed protesters with rubber bullets and tear gas while brandishing body armor and assault rifles based on the M-4 Carbine. This is a picture of police in Ferguson, from a poignant article in Business Insider:

One could be forgiven for thinking that these men do not look at all like American police officers. Indeed were it not for the "Police" sticker slapped on the front of their body armor, they could be mistaken for any arbitrary paramilitary force. Give police a soldier's armament, and you'll convince them that they are soldiers. And if they're urban soldiers, the streets become their battlefield, and everyone looks like a potential enemy combatant.

This isn't some idle, slippery-slope conjecture; it's happening. Heavily armed border police have killed 19 Mexicans for the crime of throwing rocks. The ACLU has documented seven people being killed, and 47 being injured, by unnecessary SWAT raids which, as John Stossel notes, are now being used primarily to arrest nonviolent drug offenders, with a big margin for error:
SWAT raids are dangerous, and things often go wrong. People may shoot at the police if they mistake the cops for ordinary criminals and pick up guns to defend their homes against invasion. Sometimes cops kill the frightened homeowner who raises a gun.
Stossel also argues, rightly, that this affects all of us — not just, as some conservatives would like us to believe, people behaving badly:
It took only [90 minutes] for authorities to deem [comedian Joe Lipari] a threat and authorize a raid by a dozen armed men. Yet, says Lipari, "if they took 90 seconds to Google me, they would have seen I'm teaching a yoga class in an hour, that I had a comedy show."
Lipari has no police record. If he is a threat, so are you.
But while this affects us all, in the wake of Michael Brown's death it's become clear how critical an issue this is for minorities in America. The aforementioned ACLU report found that minority communities were disproportionately targeted for these violent raids. In a sobering article for The Concourse, Greg Howard contrasts the violence against young black Americans with the hullabaloo over "open carry" laws championed by white ultra-conservatives:
There are reasons why white gun's rights activists can walk into a Chipotle restaurant with assault rifles and be seen as gauche nuisances while unarmed black men are killed for reaching for their wallets or cell phones, or carrying children's toys.
Our Constitution guarantees us the right to peaceably assemble, and the freedom of the press. This week in Ferguson, peaceful protesters and journalists alike were pelted with rubber bullets and tear gas by paramilitary police. The freedom of the press, in particular, exists to protect the interests of civilians by forcing transparency of the state. When journalists are threatened with arrest and assaulted by police, there is no one to hold police accountable for questionable actions they may take against civilians.

With surplus military gear still pouring into police departments, this trend is unlikely to change any time soon unless we stand against it. What can we do? We can call our elected officials, and partner with the ACLU. And just for the hell of it, I went ahead and created a petition at Whitehouse.gov, which you can sign here.

12 August 2014

Dan Dennett and William Lane Craig on the decline of the church

It's pure coincidence that these were released around the same time, but they both provide unique perspectives on the decline of the Christian church here in the West.

Dan Dennett: "Can churches survive the new transparency?"

William Lane Craig: "Reasons youth are leaving the church" (podcast)

11 August 2014

Update on current projects

You know how, in the past, I've said that I'm working a book (or several)? Well, I'm working on two, and they're coming along briskly. One is simply a sort of "best of", which will be called Confessions of an A-Unicornist. I've picked, with your help, my best work from the past 4½ years of this blog and I'm organizing it by topic and slightly editing the posts for flow. I honestly have no idea when it will be done, but I'm making a point to work on it a little bit every day.

The other book is on a topic that's been on my mind a lot lately, and I don't want to let out too many spoilers here but I think it's a topic that all intellectually engaged non-believers will find themselves reflecting on sooner or later. It's my primary writing project right now, which is why the blogging has been slow and will likely continue to be. It's one of those projects that started out as a very short book, but research into the subject has greatly deepened my thoughts on it.

As for the blog itself, if I can get some good content up here once a week, I'll be satisfied. Right now I want to keep focusing primarily on the books, and get 'em done! I haven't decided yet how I'll go about publishing, but one thing's for sure: they'll be dirt cheap.

09 August 2014

A brain on a chip?

In Star Trek: Voyager, the titular spaceship had computers that ran on "bio-neural circuitry" stored in gel packs. Like human bodies, the bio-neural circuity was prone to viral infection and, in one episode, is treated with a makeshift "fever" created by an "inverted warp field", because Star Trek.

How far-fetched is the idea? As it turns out, not very. Scientists at IBM have developed a chip that mimics the neural structure of a brain. Wired explains:
In a [conventional] von Neumann computer, the storage and handling of data is divvied up between the machine’s main memory and its central processing unit. To do their work, computers carry out a set of instructions, or programs, sequentially by shuttling data from memory (where it’s stored) to the CPU (where it’s crunched). Because the memory and CPU are separated, data needs to be transferred constantly.
Neuromorphic chips developed by IBM and a handful of others don’t separate the data-storage and data-crunching parts of the computer. Instead, they pack the memory, computation and communication parts into little modules that process information locally but can communicate with each other easily and quickly. This, IBM researchers say, resembles the circuits found in the brain, where the separation of computation and storage isn’t as cut and dry, and it’s what buys the thing added energy efficiency—arguably the chip’s best selling point to date.
It's an interesting concept, and as the New York Times notes, it is both power-efficient and capable of massive parallel processing:
The chip contains 5.4 billion transistors, yet draws just 70 milliwatts of power. By contrast, modern Intel processors in today’s personal computers and data centers may have 1.4 billion transistors and consume far more power — 35 to 140 watts.
Today’s conventional microprocessors and graphics processors are capable of performing billions of mathematical operations a second, yet the new chip system clock makes its calculations barely a thousand times a second. But because of the vast number of circuits working in parallel, it is still capable of performing 46 billion operations a second per watt of energy consumed, according to IBM researchers.
Large-scale applications are still a long ways off, and unlike brains and conventional computers, the new chips can't learn. So maybe we're a ways off from supercomputers bearing any resemblance to the human brain. And these new chips are a good many steps away from the Voyager computers because they just mimic an aspect of brain structure — they don't actually contain organic matter.

Still, it's kinda cool to think about. Yann LeCun, a researcher in the field, is skeptical:
“This avenue of research is not going to pan out for quite a while, if ever. They may get neural net accelerator chips in their smartphones soonish, but these chips won’t look at all like the IBM chip. They will look more like modified GPUs.”
So, like, Assassin's Creed Unity in 4k resolution on my gaming PC? I guess that'll hold me over until we build some starships.

03 August 2014

Necessary beings don't exist

As I'm prone to do, I hopped over to William Lane Craig's ironically named website Reasonable Faith last night and read the latest Q&A. This one addressed what I think remains the single most atrocious argument for God's existence — the ontological argument. The argument comes in several forms, but the theme is always the same: God exists by definition. And it still astounds me that otherwise bright people think this makes for a persuasive argument.

The Q&A discussion begins with a reader's inverse take on the the argument:
When I think about the concept of God --a maximally great being-- it seems clear that God, if he exists, exists necessarily. So if God exists in the actual world, then there is by definition no possible world in which God does not exist. But the problem is this: there seem to be a nearly infinite number of possible worlds in which God does not exist
I'll let you read the full question for yourself, but the gist is that to accept the modal ontological argument, one has to accept that there is no possible world in which God does not exist; to reject it, one merely has to accept that there is only one possible world in which God does not exist.

Craig's response is that imagining a possible world in which God does not exist "begs the question by assuming that the concept of maximal greatness is incoherent. Just because we can imagine a world in which a single particle (or whatever) exists gives no reason for thinking that such a world is metaphysically possible".

Le sigh. "Maximally great"? "Possible world"? "Metaphysically possible"? Half of the chore of addressing these arguments is deciphering the bizarre and often nebulous terminology. So let's look at the terms:

1) I don't think it's readily apparent that the concept of "maximal greatness" is coherent, because 'maximal' denotes a quantitative property, and 'greatness' denotes a qualitative one. In other words, there is no universally accepted definition of what constitutes 'greatness' in the first place, much less how greatness could be quantified as 'minimal' or 'maximal'.

2) "Possible world" is just philosopher slang for 'possible'. It seems to me then that it's utterly superfluous. If you're trying to reason about whether something is possible, just say "it's possible" or "it's not possible".

3) It's impossible to know what is or isn't "metaphysically possible" because the term 'metaphysically' is nebulously defined. Indeed Craig himself tacitly admits this in an old Q&A when he concedes, "What we take to be metaphysically necessary/possible depends on our intuitions about such matters."


The "possible world" semantics can be seen for how ridiculous they are simply by looking at one of the key premises in Alvin Plantinga's version of the argument:
4. If a maximally great being exists in every possible world, then it exists in the actual world.
In other words, "If it is possible that a maximally great being exists, then a maximally great being exists".

What? There has to be some sort of hidden premise here, because it's obviously a non sequitur to simply say "It is possible that a exists, ergo a exists". The hidden premise is embedded in the idea of a so-called 'maximally great' being. Namely, these theologians conceive of a maximally great being who exists as being greater than a maximally great one who doesn't. Confused? You ought to be. Here's Craig's explanation:
When you think about it, anything that exists must have the property of existing in every world in which it exists! So you're right that you, I, and everyone else has existence as part of his or her essence in that sense. Rather the claim here is that God exists in every possible world. What God has that we don't, then, is the property of necessary existence. And He has that property de re, as part of His essence. God cannot lack the property of necessary existence and be God. Of course, if something has the property of necessary existence, it can't lose that property, since if it did, there would be a possible world in which it lacked necessary existence and so it was never necessarily existing in the first place!
And here we find the elephant in the room: the property of existence. From the SEoP:
There is a long and distinguished line of philosophers, including David Hume, Immanuel Kant, Gottlob Frege, and Bertrand Russell, who followed Aristotle in denying that existence is a property of individuals, even as they rejected other aspects of Aristotle's views. Hume argued (in A Treatise of Human Nature 1.2.6) that there is no impression of existence distinct from the impression of an object, which is ultimately on Hume's view a bundle of qualities. As all of our contentful ideas derive from impressions, Hume concluded that existence is not a separate property of an object. Kant's criticism of the ontological arguments for the existence of God rested on a rejection of the claim that existence is a property of an object. Proponents of the ontological argument argue that the concept of God as an entity with all perfections or a being of which no greater can be conceived entails God's existence, as existence is a perfection and a being that exists is greater than a being that does not exist. Kant objected (in his Critique of Pure Reason, A596/B624-A602/B630) that existence is not a property. “Thus when I think a thing, through whichever and however many predicates I like (even in its thoroughgoing determination), not the least bit gets added to the thing when I posit in addition that this thing is. For otherwise what would exist would not be the same as what I had thought in my concept, but more than that, and I could not say that the very object of my concept exists” (A600/B628). Finally, both Frege and Russell maintained that existence is not a property of individuals but instead a second-order property—a property of concepts, for Frege, and of propositional functions, for Russell.
What these philosophers were getting at is that conceptual abstractions do not have literally real properties; their properties are, themselves, conceptual abstractions. I can say for example that a unicorn (an abstraction) has the property of looking like a horse, having a horn, being delicious when canned, etc. But these properties are nothing more than conceptual abstractions — representative processes in the human brain. I cannot claim that by adding the property of "existence" to a unicorn, a unicorn is now a real thing. It's the other way around: something has to exist in order to have properties in the first place. To put it more plainly, imaginary things have imaginary properties.

This means that just because I can conceive of a being who is 'maximally great' — however I choose to define maximal greatness — I don't have any reason to think such a being actually exists. All I've done is conjure up some imaginary thing, and its actual existence still needs to be demonstrated independently of my ability to conceive it.


Some properties of Equinas Unicornus
There are other forms of this argument: Leibniz claimed that God is a necessary being because the explanation of a series of contingent things cannot itself be a contingent thing; Aquinas claimed that 'essence' and 'existence' are identical in God, so that God being non-existent is not only paradoxical, but inconceivable.

What strikes me about all these arguments is the peculiar way in which they bandy about commonly used terms. For example, Aquinas' argument relies on a concept of 'pure being'; God, in whatever ineffable way, is not an amalgamation of properties but rather a being in which all his properties have somehow melded together and are indistinguishable from existence. It's bizarre because we've never seen anything like this, and we don't have any reason to believe that distinct properties can meld together in that way, somehow becoming identical to each other. I think Hume's argument, above, is appropriate here: there is no impression of 'existence' that is distinct from the impression of an object, which is (more or less) a bundle of qualities. We don't have any reason to think that 'pure being' is even coherent (it seems obviously paradoxical to me), but even if we did we've only conjured up a conceptual abstraction — the coherency of a concept is necessary, but not sufficient, to show that it corresponds to reality.

Why do these semantic 'proofs' of God rely on such nebulous and equivocal terms like 'metaphysical necessity', 'perfection', 'maximal greatness', and 'pure being'? Sean Carroll nicely captures the allure of this type of convoluted thinking:
If you have God intervening in the world, you can judge it by science and it’s not a very good theory. If on the other hand God is completely separate from the universe, what’s the point? But if God is a necessary being, certainly existing but not necessarily poking into the operation of the world, you can have your theological cake without it being stolen by scientific party-crashers, if I may mix a metaphor. The problem is, there are no necessary beings. There is only what exists, and we should be open to all the possibilities.
The simplest and most rational view is that the entire concept of necessary beings is inane theological gobbledygook. We don't have any reason to think that existence is something that can be ascertained in a way distinct from an object that is itself an amalgamation of observable properties. We don't have any reason to think that existence can be a property of something, or that 'maximally great' is anything more than a theological conjecture dependent upon idiosyncratic definitions of terms. Most importantly, we don't have any reason to think that our mere ability to conjure up a seemingly coherent concept is reason enough to think that it corresponds to reality. As Carroll himself often says, we can't know what reality is just by thinking about it; we can contemplate possible ways it could be, but eventually we have to actually get out there and look.