"And God created man to his own image: to the image of God he created him: male and female he created them." – Genesis 1:27
"λόγον δὲ μόνον ἄνθρωπος ἔχει τῶν ζῴων." – Aristotle, Politics
If someone asked you what a hammer is, it would do little good to say "it's a tool" and stop there. There are many types of tool. Saws, sickles, screwdrivers, spades, squeeze clamps, studfinders, squares, steelyards... and that's just "S." Saying a hammer is a tool does little more than tell you the general category into which the item falls, what sort of object it is generically; this is therefore called its genus.
But to understand what a hammer is, you need more than its genus. The genus may be tool, but you need to narrow it down. I could say many things about the hammer hanging in my toolshed, but which of those would help to define it? If I said, "it's a tool and is blue," that would be true, but is that what makes it a hammer? If I said, "it's a tool and weighs 12 ounces," that would also be true, but is that what makes it a hammer? Of course not. Although they are true statements about the tool, they are not essential to its being what it is; they are not part of the hammer's essence (facts that are not essential facts are called accidents.)
So how do we get at the hammer's essence? It would take a statement like, "it's a tool with a handle and one heavy end for hitting things with lots of force." This gets us past the genus and, instead of wasting time with non-essential facts, gets to the essence of what it means to be a hammer by explaining what kind of tool it is specifically. This is called the specific difference or species. The species, then, defines the essence of a thing by showing how it differs from other members of its genus.
So what are the genus and specific difference of a human person?
We are certainly animals; without going into too much detail, that's our genus. But as G.K. Chesterton said, hitting the nail of genus on the head with the hammer of species, "That man and brute are like is, in a sense, a truism; but that being so like, they should then be so insanely unlike, that is the shock and the enigma." A fancy way of saying that genus is not enough. We must get to the specific difference.
Going a bit deeper, scientists divide animals into different kingdoms, phlya, classes, orders, families, and genera (which is the plural of genus) although philosophically, each of these divisions could be considered either a genus or a species. To use the scientific terminology, our genus is homo or "man," which consists of us plus several now-extinct creatures that looked very similar to us. The question, then, is our specific difference: what makes us different from the other homoi?
The astute reader will have already picked up that the taxonomical name of our species is homo sapiens, or "knowing man." From Ancient Greece and even before, this has long been accepted as the primary difference between us and any other animal we have ever encountered. Many animals are quite clever, but they do not have "knowledge" in the proper sense. They do not consider concepts, weigh propositions, or reason through abstractions. Lots of animals can pass a "mirror test," but only one animal has ever devised, conducted, and analyzed the results of a mirror test. It's also important to note that this is a difference in kind, not in degree. If it were only a difference in degree, we could expect monkeys to do arithmetic and chimpanzees to do algebra while only humans could dabble in calculus. In reality, of course, even uneducated humans are capable of thinking mathematically, whereas even highly trained orangutans cannot count. The fact that some researchers have managed to get apes to replicate this behavior in some way has only heightened our conviction that the animals do not understand what they are doing, even if they imitate the process.
To illustrate this, The Sons of God considers the idea of a homo asapiens, or a "wild man," a human being not made in the image of God, without the rational essence of human nature. In this passage from Chapter 7, the narrator interacts with a girl, whom he has named "Sparrow," who, though physically human, is not a descendant of Adam, and lacks the rational nature that is the essence of humanity.
Gritting my teeth, I turned back to Sparrow. She was perched atop the stone, knees up by her shoulders, arms drooping down in front of her. Every few moments, her head flitted in a new direction, turning now to me, now to the birds, now to the horizon.
“What are you looking for?” I asked, but there was no sign of understanding. “Where shall we go next?” I tried. It was as if she had not heard me. “You found this water. Do you know where we can find more food?”
“Food!” she chittered in her high, bird-like voice. She shook with excitement. My brow furrowed. Maybe she could speak, but no one had taught her. I would try. I pointed to the water.
“Water,” I said.
“Watah,” she repeated. “Good,” I said, smiling at her. But then I saw her eyes. She was looking at my hand, not at the pool. I moved my hand to point at the stone on which she was sitting. Her eyes followed.
“Stone,” I tried.
“‘Tone,” she answered, still looking at my hand. Now came the test. I pointed to the water again and looked back at Sparrow.
“Tone,” she said. “‘Tone, ‘tone, ‘tone!"
With a heavy sigh, I tossed my hands into the air. Sparrow blew air through her lips and mimicked me, throwing her hands up as I had. I frowned and looked back to the water hole. I scanned the area, wondering which was the best way for us to continue on.
Possessing an anatomically modern human body, Sparrow has a fully developed brain and all the prerequisite physical requirements for speech; but lacking a rational nature, she is not capable of abstracting concepts, forming propositions, or reasoning to conclusions. She can imitate the sounds of language and respond to sounds, but only as signs, not as symbols. She has learned, in a Pavlovian way, that the sound "food" means she can expect a meal soon, but cannot make the leap that the sound "food" means or refers to food. She can mimic complex behaviors, but would not understand their significance. She is capable of affection, but would not be capable of love in the proper sense, which is an act of the human will.
I cannot claim this exercise is unique to my novel; an excellent of example of it is Jack London's treatment of the actions of the dogs and wolves in The Call of the Wild and White Fang. London often takes care to point out that the animals do not think or consider their actions, they simply act on instinct and do whatever comes to them at the moment. Applying this concept to a physically human character, we can see explore human nature more deeply.
Human nature – literally the meaning of life – is so important to understand. It might seem like a boring, abstract idea, but getting as it does to the essence of mankind, it is an idea with very practical and wide-ranging application, and horrifyingly devastating consequences if we get it wrong.