The first step of the GTD workflow is about collecting everything that catches your attention; any input that might be meaningful to you or that is already meaningful to you, even if you are not doing anything about it yet.
Capturing does not mean interrupting what you are doing to do something else. It just means reacting to something that comes to your mind and writing it down, without spending more time on it at that moment, so that you can continue with what you’re currently focusing on without being distracted by anything else.
In order to manage this inventory of incomplete items you need to collect them in a reliable external system until it’s time to decide what they are and what to do with them. The purpose of capturing is to make sure that everything you need is out of your head and accessible.
A good practice is to capture 100% of your incomplete inputs, even those that seem unimportant, so that your organizational system represents an accurate picture of your real commitments and interests. If your system does not match your reality, your motivation will soon wane.
The mind sweep
If you are just starting to use GTD, or you have fallen off the board, or you still haven’t fully acquired the habit of capturing, the “mind sweep” is a very useful capture technique that you can use every time you have the feeling that there are still things outside your organizational system.