Full Explanation
The context window is the amount of information a model can see at one time. It's not memory — it's working space. The model can only reason about what is currently visible. At any moment, it receives a fixed block: your message, some recent messages, some instructions. That block is its entire world.
If something is inside that block, the model can reason about it. If something is outside, it does not exist — not ignored, not forgotten, just invisible.
A useful metaphor is a desk. Only the papers currently on the desk can be used. As new papers arrive, old ones slide off the edge. Once they fall off, the model cannot look at them anymore.
This explains why instructions seem to disappear, why answers contradict earlier statements, and why long conversations slowly fall apart. The model isn't being careless — it simply no longer sees what you think it should remember.
The key insight: reasoning only happens over what is visible. The model does not reason over the past — it reasons over the current context.
