in Thoreau’s Journal:

I feel a little alarmed when it happens that I have walked a mile into the woods bodily, without getting there in spirit.  I would fain forget all my morning’s occupation—my obligations to society. But sometimes it happens that I cannot easily shake off the village—the thought of some work—some surveying will run in my head and I am not where my body is—  I am out of my senses.  In my walks I would return to my senses like a bird or a beast. 

What business have I in the woods if I am thinking of something out of the woods.