Resolving this Project in my Mind, as to be undertaken hereafter when my Circumstances should afford me the necessary Leisure, I put down from time to time on Pieces of Paper such Thoughts as occur'd to me respecting it. Most of these are lost (3.4)
Again, Franklin knows his memory is fallible, so when he has some interesting ideas about his project, he decides to write them down. He thinks then he'll come back to them later, so he decides to keep his "Thoughts" outside of himself on paper – kind of like how Franklin puts his life's memories into the Autobiography. What's too bad is that these papers are just as fallible as his memory, and when he loses the papers, his ideas are doubly lost.