You will find a mark [1] after that phrase. It refers to a footnote. Glance (or look) at the bottom of the page and you will find:
[1] But if one does one is a fool.
So I slept late and uneasily the insufficient sleep of men who have suffered, and in that uneasy sleep I discovered this great truth: that if in a southern
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WHY 'DECIMO'?
summer you do not rest in the day the night will seem intolerably warm, but that, if you rest in the day, you will find coolness and energy at evening.
The next morning with daylight I continued the road to Lucca, and of that also I will say nothing.
LECTOR. Why on earth did you write this book?
AUCTOR. For my amusement.