Author: Maryh
All quotes are from "A Magical Worldview" by Steve Vander Ark at HP Lexicon.
In the muggle world, power over the physical world is a question of engineering and physics; they are not personal, and have nothing to do with a person's intent or desire. It doesn't matter how strongly a muggle wants to float gently down to the bottom of the Grand Canyon after jumping off the edge -- the law of gravity states how fast he is, in fact, going to fall.
The wizarding world is quite different. " ... magic is really the manipulation of magical power by the intention and focus (by means of a wand and words, usually) of the caster. The exact physics of the spell--how it works, how it's cast, what it does--can be different every time, because physical laws aren't involved, emotions and intentions are."
In the muggle world, those who excel in the application of impersonal logic attain the most power over the physical world. In the wizarding world, those who are best able to focus their mind and, for certain spells, the appropriate emotions, on what they want to happen, attain this kind of power.
Although words and a wand are usually used to focus the direction of the magical power, "In its purest form, magic requires no words and even no wand. We see Dumbledore using magic this way, or close to it, almost every time he does it." For a powerful wizard, floating rather than falling down the canyon has everything to do with how much he wants to float rather than fall.
The muggle scientist and engineer attain control over the physical world by suppressing emotions, and concentrating on logic to build tools. The wizard and witch attain control over the physical world by having ready access to their emotions, and by having a rich stock of different emotional thoughts to draw from, in order to perform spells.
In most of the spells that we see in the series, the primary emotional content is simply the intent. For the spell to succeed, the witch or wizard has to want whatever it is that the spell does, and focus the mind on this desire. But more complex spells require more emotional skill.
A happy thought, properly focused, produces a patronus to repel a dementor. Hatred, properly focused causes death. And we have hints that love, properly focused, can repel an AK curse. But the wizard has to have experienced the relevant emotion, must have access to it when necessary, and must be able to focus his mind on it.
Let's take a look at a more complicated spell, the Riddikulus spell to repel a boggart. It requires quite a bit of emotional self knowledge and control, and even a competent adult witch like Molly was unable to perform it at one point in The Order of the Phoenix because, at the time, she could not muster the necessary emotional control. First of all, the witch must know what her worst fear is. Secondly, she must be able to face that fear directly enough to be able to manipulate her image of it. Thirdly, she must know what kind of thing would make her laugh. Molly's worst fear at the time was the death of her loved ones. She had to be able to face that fear, and then make it look funny.
With this understanding of magic, it becomes more apparent that of all the houses, the Slytherins would be most tempted to power. Slytherin, the house related to the astrological element of water, is guided by emotion and intuitive knowledge or "hunches"; that is, they are most attracted to the very same thing that happens to be most important in gaining power over the physical world if you are a wizard or witch.
In the muggle world, the Ravenclaws, with their love of knowledge and cleverness, would become the scientists and engineers who apply logic to create the atom bomb. In the wizarding world it is the Slytherins, with their emotional self knowledge and control, who practice the dark arts and produce a wizard like Lord Voldemort.