in Thoreau’s Journal:

This afternoon, late & cold as it is has been a sort of Indian summer. Indeed, I think we have summer days from time to time the winter through, and that it is often the snow on the ground which makes the whole difference. This afternoon the air was indescribably clear—& exhilarating—& though the thermometer would have shown it to be cold I thought there was a finer & purer warmth than in summer.  A wholesome intellectual warmth in which the body was warmed by the mind’s contentment—  The warmth hardly sensuous but rather the satisfaction of existence….