The hypothesis reappeared in 1786 when Sir William Jones first lectured on the striking similarities between three of the oldest languages known in his time : Latin , Greek , and Sanskrit , to which he tentatively added Gothic , Celtic , and Old Persian , though also commiting some inaccuracies and omissions in his classification . The hypothesis reappeared in 1786 when Sir William Jones first lectured on the striking similarities between three of the oldest languages known in his time : Latin , Greek , and Sanskrit , to which he tentatively added Gothic , Celtic , and Old Persian , though also committing some inaccuracies and omissions in his classification .