What Thoreau is saying is so important to you today. Be willing to dream, and imagine yourself becoming all that you wish to be. If you live from those imaginings, the universe will align with you in bringing all that you wish for—and even more than you imagined when you were living at an ordinary level of consciousness, or what Thoreau calls "common hours." Keep in mind the basic axiom—all that now exists was once imagined. It follows then that what you want to exist for you in the future must now be imagined.
You'll see as you read through these foundations for fulfilling your wishes that using your imagination involves more intensity on your part than just imagining an occasional wishful thought. You must first unlearn some of the ways that you've been misusing or squandering the capacity your imagination has for providing you with an unlimited ability to manifest all that you desire. Thoreau called it advancing confidently in the direction of your own dreams. Unfortunately, most of us have learned to do the opposite with our imagination, and instead regularly use it to develop mental habits of advancing in precisely the reverse direction of our highest dreams for ourselves.