UX is how it feels. UI is what you’re actually looking at and clicking on.
Stuart Card, Thomas Moran and Allen Newell’s 1983 The Psychology of Human-Computer Interaction is one of the foundational texts of the field now called HCI — it modelled how humans process information while operating a computer, treating interface design as an applied psychology problem rather than a purely technical one. That framing — the interface as the negotiated boundary between human cognition and machine logic — is why UI and UX get discussed together but aren’t the same thing: UI is the artifact (the buttons, the layout, the screen), UX is the judgment of how well that artifact serves the person using it.
This essay’s closing line pairs them as adjacent, not identical, concerns — “efficiency and ease of use and user interface that will matter eventually” — because a business evaluating an AI tool eventually has to judge both: does the interface make sense, and does using it actually feel good.
Source: Card, S.K., Moran, T.P. & Newell, A. (1983), The Psychology of Human-Computer Interaction, Lawrence Erlbaum Associates / CRC Press.