During the Renaissance, the five senses were understood to form a hierarchical system from the highest sense of vision down to touch. Vision was correlated to fire and light, hearing to air, smell to vapour, taste to water, and touch to earth.
Perhaps the human fascination with fire stems from the totality of its sensory stimulation. The fire gives a flickering and glowing light, ever moving, ever changing. It crackles and hisses and fills the room with the smells of smoke and wood and perhaps even food. It penetrates us with its warmth. Every sense is stimulated and all of their associated modes of perception, such as memory and an awareness of time, are also brought into play, focused on the one experience of the fire. Together they create such an intense feeling of reality, of the "here and nowness" of the moment, that the fire becomes completely captivating.
People mix the terms UI and UX together. UX is tricky because it doesn’t refer to any one thing. Interface design, visual styling, code performance, uptime, and feature set all contribute to the user’s “experience.” Books on UX further complicate matters by including research methods and development methodologies. All of this makes the field confusing for people who want to understand the fundamentals.
That’s why I avoid teaching the term ’UX.’ It means too many things to too many different people. Instead I focus on individual skills. Once you understand the individual skills, you can assemble them into a composite system without blurring them together. For software design, the core skill among all user-facing concerns is user interface design.