Yesterday is an abstract concept in the sense that events that occurred in the past do not exist in the present reality, though their consequences persist. An often-quoted aphorism states something along the lines of "Yesterday is history, tomorrow is a mystery, but today is a gift. That is why it is called the present".[4] The earliest version of this aphorism appears to occur in 1912, with the Frank M. Pixley quote, "Yesterday is history; to-morrow is merely a hope; to-day is the only absolute asset of time that is yours".[5]