Something small happens. A passing comment, a delay, a certain tone. And the reaction inside is out of all proportion: hurt, flooded, thrown. You decided long ago how you want to be, and yet here is the old response again, as if your past had taken the wheel.
The course explains this with impressions. Every action and experience leaves a trace, and together those traces make up your personality, like an iceberg mostly hidden below the surface. Beliefs form the same way, out of past experiences, conversations and influences. When something touches an old impression, it rises, floods the mind and overrides the decision you made.
It's tempting to want to tip the old pattern out and start clean. The course says that can't be done, and offers a picture instead. Hold a glass of dirty water under a running tap of clean water and it overflows, the dirt thins, and in time it runs clear. Old patterns change the same way: by adding fresh thoughts, again and again, until what's left, the course says, are your original qualities of purity, peace, love, truth and joy. It needs a steady tap, not one big flush.