The interface is the visible edge of the product.

A serious product often depends on a layered stack — state, memory, permissions, timing, confidence, orchestration, recovery, visibility — that is doing much of the work. Memory has provenance, scope, freshness, expiry, correction, and deletion. Confidence needs measured evidence. Permission remains separate from both. To design a product is to design those layers and then let the visible surface express them clearly. Sketch early if it helps you think; the interface is often how the system becomes understandable. But do not let the screen become the only source of truth.