Philosophers have been arguing that babies are born with minds that are essentially blank slates since the days of Aristotle. (Later, some psychologists took up the case as well.) English speakers have called that initial state of mental blankness tabula rasa (a term taken from a Latin phrase that translates as "smooth or erased tablet") since the 16th century, but it wasn't until British philosopher John Locke championed the concept in his Essay Concerning Human Understanding in 1690 that the term gained widespread popularity in our language. In later years, a figurative sense of the term emerged, referring to something that exists in its original state and that has yet to be altered by outside forces.
1535, in the meaning defined at sense 1
Latin, smoothed or erased tablet