Perception is Everything

Zedd led them to the edge of the grass. They walked between the long shadows of the trees to the place where two little thorns grew, one as thick as Richard's fist, the other as thin as Kahlan's.
He turned to Richard. "Draw the Sword." The special, ringing, metal noise filled the late noon air when he drew it. Zedd leaned further forwards. "Now I will show you the most important aspect of the sword, but to do that you have to give up your position as Seeker for a short while and allow me to name Kahlan the Seeker."
Kahlan looked suspiciously at Zedd: "I don't want to be a Seeker."
"Only for this demonstration, my dear." He gestured Richard to give her the sword. She hesitated before she took it with both hands. The weight was unbearable and she lowered the tip on the grass. Zedd waved his hands above her head: "Kahlan Amnell, I name you the Seeker." She kept her eyes on him with the same suspicious look. Zedd put his finger under her chin and lifted her head. His eyes were wild. He brought his face closer to hers and spoke with soft voice.
"When I left the Midland with this sword, Darken Rahl put these two trees here with his magic. To mark me, to be able to come and get me when he chooses to. The same Darken Rahl that had your sister killed." Her expression became darker. "The same Darken Rahl that is following you, to kill you like he did Dennee." Hate was rising in her eyes. She clenched her teeth so the muscles in her strong jaw became visible. The Sword of Truth rose from the ground. Zedd went to stand behind her. "This tree is his. You have to stop him."
The sword flashed through the autumn air with a speed and power that Richard barely held possible. A loud cracking sound went through bigger tree, as if a thousand branches broke at once. Splinters flew everywhere. The tree seemed to be momentarily suspended in air, and then fell with a boom (boom, flemish/dutch for tree ;p) next to what was left of it in the ground.
Richard knew he would have needed at least ten good axe strikes to do the same.
Zedd let the sword glide out of Kahlans hands. She sank to her knees, and put her hands in front of her face. Immediately Richard squatted next to her to support her.
"Kahlan, what's the matter?"
"I'm ok." She put a hand on his shoulder and he helped her up. Her face was pale, but she tried to smile anyway. "But I distance myself from my position as Seeker."
Richard turned to the wizard. "Zedd, what is this for nonsense? Darken Rahl didn't put that tree there. I've seen you water those trees and care for them. If you'd put a knife to my throat, I would say that you planted them to remember your wife and daughter."
Zedd smiled only mildly. "Very good, Richard. Here is your sword. Now you are the Seeker again. Now you cut down the small tree, my boy, and I will explain."
Frustrated Richard took the sword in both hands; he felt the anger surge through him again. He directed the sword with a powerful swing up towards the remaining tree. The tip of the sword cut through the air whistling. Just before the sword hit the tree, it suddenly stopped, as if the air around it had become too thick to let it through. Richard stepped back amazed. He looked at the sword and tried again. Once more the sword stopped. The tree wasn't hit. He looked at Zedd in a fury who was grinning with folded arms.
Richard put the sword away. "Ok, what's going on."
Zedd innocently raised his eyebrows. "Have you se how easily Kahlan cut through the big gtree?" Richard frowned. Zedd smiled. "He could have been made of iron. The sword would have gone through it just as easily. You are stronger than Kahlan and you couldn't even scratch the little tree."
"Yes, Zedd, I noticed that."
Zedd's forehead wrinkled as if he was amazed. "And how is that, do you think?"
Richard's frustration melted away. This was the way in which Zedd always taught him, by letting him give the answer himself. "I would say that it had something to do with the intention. She thought the tree was evil, I didn't."
Zedd stuck a bony finger in the air. "Very good, my boy!"
Kahlan reacted nervously. "Zedd, I don't get it. I destroyed the tree, but it wasn't evil. It was innocent."
"That, my dear, is what this demonstration is about. Reality is not relevant. Perception is everything. If you think it is the enemy you can destroy it, wether true or not. The magic interprets only your perception. It will not harm someone you deem innocent, but it will destroy everyone you see as an enemy, within limits. Only what you believe is the determining factor, and not if those beliefs are true.
Richard was somewhat overwhelmed. "That doesn't leave any room for mistakes. What happens if you're not sure?" Zedd lifted an eyebrow. "I'd better be sure, my boy, otherwise you could get a heap of troubles. The magic could read things from your mind where you are not even conscious of. It could be dual. You could kill a friend, or not kill an enemy.

Excerpt from Terry Goodkind's The Sword of Truth 1: Wizard's First Rule.