Three years ago. Lecture hall 102 in the Humanities Building at Geumseong University.

Oreum stood at the lectern in a neat shirt, knit sweater, and pants. Beneath the glasses frame that had not quite hidden them, there was a tired shadow under his eyes.

It was the first day back after a long sick leave.

His leather wristwatch, loosened by lost weight, kept sliding down below his wrist bone.

"Hello. I'm Seokdal Oreum, and this semester I’ll be teaching <Brain and Mind>...."

Oreum cleared his throat once and adjusted the microphone position unobtrusively.

The medium-large general education lecture hall for about 100 students was already full of students' heat and faint noise. The tiered seats, gently sloping downward, spread out in a semicircle, and the farther back they went, the higher the view rose, surrounding the lectern like the stage of a circular theater. The fluorescent lights evenly hung from the ceiling poured down over the students' young heads, and the projector hanging in the center spun slowly, making an old fan noise.

Most of the students in front of him were in their early twenties. The sounds of laptops or tablets turning on burst out one after another. Squeaking hinges, a faint blue glow spreading thinly. Some of them seemed to already be skimming the PDF lecture materials he had uploaded.

Even though the room was large, once hundreds of gazes gathered together, the air quickly grew stuffy. Someone's cough, the wheel sound of a chair being pushed back slightly, the rustling of someone seeming to write on a pad all piled up in Oreum's ears.

Oreum unconsciously looked down at his wrist. The watch band kept turning. But he could not tighten the strap in front of the students. It felt like a vulnerable action. More than anything, Oreum was too tired.

As the projector screen changed, the lecture title appeared.

<Brain and Mind>

For an instant, the lecture hall quieted down. Standing between the bright light of the screen and the dark shadow behind the lectern, Oreum became aware that he was holding out alone in that ambiguous boundary. He gathered his breath once more and opened the introduction.

"Evaluation will be attendance 20 percent, the midterm and final exams 30 percent each, and one short report 20 percent. Attendance will be checked automatically through eTL at the start of class every week, so please don't forget to check it. There is no designated textbook. You can bind the materials I upload every weekend or bring them loaded on your pad. If you have questions, please send me an email, or use the bulletin board. And...."

Oreum hesitated a little, then went on stiffly with the notice.

"Professor is a title, not a form of address. I am not a full professor, so I would be grateful if you called me 'teacher' in class or outside."

For Oreum, that was almost like a necessary ritual. It was also a kind of spell, a way to remind himself that his calling lay in the pure act of teaching and passing things on. The students already seemed to have relaxed, murmuring among themselves. Oreum turned over the orientation materials with a calm gesture.

"We tend to understand the mind mainly as an abstract and romantic concept. We express emotion and will in linguistic metaphors, like, 'That person has a warm heart,' or 'My heart feels heavy today.' But every phenomenon of the mind we feel is in fact directly tied to the physical activity of the brain. The way certain neural circuits are activated or inhibited, the tiny differences in concentration of neurotransmitters moving between synapses, the speed at which the frontal lobe responds to outside stimuli, these things appear in the form of emotions like 'love' or 'anger' that we feel. Feeling sad while remembering the past, understanding what I am saying to you now, and even the consciousness itself that realizes I am awake in this lecture hall right now, all of it is a phenomenon created by the exquisite biological mechanism called the brain. In the end, the 'mind' is like software running on the hardware called the brain."

Oreum covered his mouth with his hand and cleared his voice. Several students in the front row exchanged looks of interest with one another. Buoyed by that small response, Oreum took out something a little deeper.

"The reason this lecture is titled <Brain and Mind> is clear. We are going to connect sensory and subjective experience with the cold biological principles hidden behind them. Instead of reciting love or sorrow as poetry, we will learn how to explain them in a far clearer and quantified language."

Oreum flipped the screen and brought up complex graphs and EEG waveforms.

"What you are looking at here is brain waves, that is, EEG1). It is a method of attaching electrodes to the scalp and recording the brain's tiny electrical signals in real time. It is the fastest indicator for checking which part of the brain reacts, and when, to a given stimulus. EEG is noninvasive, so it is safe, and above all, useful for checking 'response speed.' The report topic this semester will also be interpreting this EEG experimental data."

He lifted the laser pointer and pointed at one steep peak in the waveform. The red dot of the pointer trembled along with Oreum's fine hand.

"This point marked right now shows the response delay to a stimulus, that is, the latent period. Latent period means the physical time it takes from the moment a stimulus enters from outside until the brain shows a meaningful electrical response. No matter how fast the human brain is, there is always a delay between stimulus and response. In the case of visual stimuli, an initial response appears on average somewhere around 100 milliseconds. If there is a problem with cognitive function or unnecessary resistance occurs in the neural transmission pathway, this latent period becomes delayed by several dozen milliseconds. We infer the brain's information-processing ability through that tiny difference."

Oreum paused for a moment and looked around at the students. Everyone seemed focused.

"The fact that the latent period is constant is evidence that the brain is communicating smoothly. But if this delay grows longer, it means that unnecessary friction or collision is occurring somewhere. The cause may be aging, it may be illness, or it may be emotional overload. For now, just keep the concept at this level."

The lecture hall was astonishingly quiet. Oreum felt that he wanted to finish his work quickly and rest.

"This is a very basic concept in neuroscience and experimental psychology. Today I introduced only the principle, and next week I will bring the equipment myself and give a demonstration. Participation will be available on a first-come, first-served basis for one person only, and before the demonstration you must fill out the research ethics consent form."

As the end of the lecture came into view, the students' behavior split into a hundred different forms. From students taking down Oreum's words without missing a single syllable, to students already zipping up their bags to prepare to leave, to those who lay sprawled on their desks and stared at Oreum with soulless eyes. The sound of recording the materials with smartphone cameras, the busy fingers of those secretly enjoying games behind their screens. Assorted desires and lethargy, tiny admiration and terrible indifference pooled beneath Oreum's feet.

Oreum suddenly felt thirsty. Since this was the first lecture he had taken on after returning, the tension would not leave him easily. In particular, a part-time lecturer was neither professor, nor assistant instructor, nor student, and did not fully belong anywhere. Eternal temporary, nothing more and nothing less. An empty space could not be filled.

Just as Oreum was about to call himself a teacher, the back door of the lecture hall opened. The cold temperature outside seeped into the room for a moment, then vanished, and someone walked in carelessly.

The student who came in late was immediately noticeable because he wore a fairly long coat and was extremely tall. He had an impression somewhere between boy and young man. The contours from forehead to jawline were sharp, and because of the angle of the lighting, a deep shadow lay around his eyes, making his expression hard to read.

That student stood leaning slightly against the wall beside the back door, showing no sign of looking for an empty seat. He had nothing with him, no bag, no writing tool. With his arms crossed in a crooked posture, he looked at Oreum on the lectern as if observing him. Oreum quietly lowered his head, but the gaze focused entirely on Oreum alone was not something he could simply brush off.

Does he know me?

Even if he used the moment to check his memory, it was clearly their first meeting. Oreum easily gave up on guessing. In any case, it would be over soon. He wanted to close the lecture, so he gave his final reminder.

"I'll upload next week's lecture materials by Sunday evening. Students who want to take part in the demonstration, please let me know in advance. Thank you all for your hard work today."

The students began one by one to prepare to leave the lecture hall. But that student, that one person, did not react at all to Oreum's words. It seemed as if he just kept observing Oreum. In that instant, Oreum realized that most of the students in the lecture hall were secretly looking at him, pretending not to know. It looked as though he had had the control of the space taken away by a mere latecomer. Oreum felt a slight sense of crisis. His heart beat one count late.

At the same time Oreum was about to close his handout, that student casually raised his hand. A slow movement. Relatively dark skin, a strange smile hanging at the corners of his mouth.

"Professor, no, teacher."

"Yes....?"