On a project.
If it were a jig-saw puzzle, there would be a rough but changing idea of the puzzle's picture as well as an infinite number of pieces. The trick is to know when to call a halt and complete the assembly but that can also be devilish because the pieces can not only complete the picture, they can change it. A new one may change it considerably.
It is important to jot down the overall concept and to note how everything might fit together. What seems obvious today may not be so tomorrow.